How to Choose the Right Mailing Box or Shipping Carton: The Ultimate Guide (2026)

Choosing the wrong box costs you twice. Too big and you pay to post air, plus the void fill to stop your product rattling around. Too small or too weak and you pay for the replacement, the return postage and, worst of all, a customer who may not order again.

This guide brings everything together in one place. Whether you send a few eBay orders a week or pack hundreds of eCommerce orders a day, follow the steps below and you will land on the right mailing box or shipping carton every time.

Quick answer: the 7-step box selection checklist

1. Know your product – weight, fragility, shape, value and how many items go in each order.
2. Pick the format – satchel, mailer, mailing box or shipping carton.
3. Pick the style – die-cut mailer, tuck front, self-seal, regular slotted carton (RSC), long box or letter box.
4. Size it properly – measure, add the right clearance, then match to the box's internal dimensions.
5. Check the strength – flute, single vs double wall, and the weight the box needs to carry and stack.
6. Check the postage – cubic weight, carrier size limits and whether the box fits a prepaid satchel.
7. Test it – pack a real order, seal it, shake it and drop it before you buy in bulk.

Now let's go through each step in detail.

Step 1: Start with the product, not the box

Before you look at a single box, write down five things about what you are sending:

• Packed weight – the product plus any retail packaging and inserts.
• Fragility – could it break, dent, scratch, bend or leak? Glass, ceramics, electronics and liquids need far more protection than clothing, books or soft toys.
• Shape – flat, cube, long and thin, or irregular. Measure at the widest points, including pumps, handles, tags and lids.
• Value – the more it costs to replace, the more a stronger box and extra cushioning pays for itself.
• Order mix – do customers usually buy one item or several? Your most common order combination should drive your core box sizes.

Step 2: Satchel, mailer, mailing box or shipping carton?

Not everything needs a box. Here is how the main formats compare:

For a deeper comparison of bags versus boxes, see Satchels vs Boxes vs Mailers.

Mailing box vs shipping carton: what's the difference?

People use the terms loosely, but they are built differently:

• Mailing boxes are die-cut from a single sheet and fold into a box with a built-in lid, so there is no tape across the top. They look neat, open nicely and are ideal for direct-to-customer orders. Ours are typically made from thin, smooth E-flute board, which prints well and keeps the box light.
• Shipping cartons (Regular Slotted Cartons, or RSCs) have four flaps top and bottom that you tape shut. They use the least material per litre of space, stack strongly and are the workhorse for heavier goods, wholesale and storage. Most of our regular shipping cartons are Australian made in C-flute board. Learn more in What Are Regular Slotted Shipping Cartons.

Step 3: Choose the right box style

• Die-cut mailing boxes – the all-rounder for eCommerce. Quick to fold, no glue, available in brown kraft, white and a wide range of colours.
• Premium tuck front mailers – a heavier D-flute board and a tuck-in front flap for a more premium, gift-style unboxing.
• Self-seal mailing boxes – a peel-and-seal adhesive strip means tape-free closing and faster packing, and some include a tear strip for easy opening.
• Regular shipping cartons (RSC) – best value for heavy, bulky and multi-item orders.
• Heavy duty mailing boxes – extra strength for weighty or high-value goods. See When to Use Heavy Duty Mailing Boxes.
• Long mailing boxes – a stackable, crush-resistant alternative to postage tubes for posters, rolled artwork, umbrellas and long parts.
• Superflat letter boxes – slim boxes for thin products that need more protection than an envelope.
• Book wrap mailers – wrap tightly around books and flat items. See The Ultimate Guide to Shipping Books.

Comparing the three main mailer styles side by side? Read our Mailing Boxes Buying Guide: Diecut vs Tuck vs Self-Seal.

Step 4: Get the size right

Measure the product correctly

Measure length, width and height at the widest points, in millimetres. If the product sits in retail packaging that ships with it, measure the retail pack. For irregular shapes, imagine the smallest rectangular block the item would fit inside and measure that.

Always match to internal dimensions

Box sizes on our store are internal dimensions unless the listing says "External". Internal dimensions tell you what fits inside; external dimensions are what the carrier measures and charges for, and are a few millimetres bigger depending on board thickness. Our guide How to Measure a Box Correctly explains this in detail.

Add the right clearance

So the minimum internal size you need is:

Product length + (2 × clearance) × product width + (2 × clearance) × product height + (2 × clearance)

Then choose the smallest standard box that meets or slightly exceeds all three measurements. If you are slightly over on one dimension, try rotating the product before jumping up a whole size.

Too big is as bad as too small

• Too small: the product presses against the walls, corners take the impact, flaps bulge and seams split.
• Too big: the product moves, you use more void fill, the walls have nothing behind them so they crush more easily, and you pay more postage for the extra volume.

Oversized packaging also annoys customers. In UK research by packaging maker DS Smith, around one in five shoppers said they were frustrated by boxes that were bigger than needed.

Step 5: Choose the right strength

Flute types made simple

Corrugated board is two flat liners with a wavy "flute" glued between them. The bigger the flute, the more cushioning and stacking strength; the smaller the flute, the smoother the surface and the better it prints.

Want the full detail? Read What's Corrugated Flutes?

Single wall or double wall?

This lines up with what the major carriers recommend: DHL, for example, advises double- or triple-wall boxes for heavy or fragile items, and TNT says single-wall boxes suit only light, non-fragile goods.

ECT and burst ratings: what the box stamp means

Quality cartons often carry a box maker's stamp showing their strength rating. You will usually see one of two numbers:

• Edge Crush Test (ECT) – how much load the board can take on its edge before crushing. This predicts stacking strength, which matters in delivery vans, warehouses and on pallets.
• Burst (Mullen) test – how much pressure it takes to punch through the board face. This relates to puncture resistance and rough handling.

For a single parcel thrown into a van, burst and puncture resistance matter a lot. For cartons stacked in a warehouse or on a pallet, ECT matters most. The two ratings are not interchangeable, so compare like with like. More detail in our Shipping Box Strength & Durability Guide.

Watch out for humidity and overhang

Cardboard absorbs moisture. Research on corrugated containers has found that changes in relative humidity can take away up to around half of a box's stacking strength, which is worth remembering in humid Queensland summers, cool rooms and damp storage. Boxes overhanging a pallet edge also lose strength quickly. Store cartons flat, dry and off the floor, and stack them squarely.

Step 6: Protect what's inside

The box is only half the job. What stops the product moving is just as important.

• Bubble wrap – the go-to for fragile items. Small bubble (10 mm) for surface protection, large bubble (20 mm) for cushioning. See our Bubble Wrap Guide.
• Kraft and packing paper – a plastic-free void fill that crumples into the gaps and is easy for customers to recycle.
• Packing peanuts – fill irregular spaces quickly in larger cartons.
• Tissue paper – for presentation and light scratch protection in mailing boxes.
• Box-in-box – for very fragile or valuable items, pack the item in a small box, then float that box in a larger carton with cushioning on all sides.

Seal it properly

For taped cartons, use the H-taping method: one strip along the centre seam and one strip across each edge seam, top and bottom. Use a proper packing tape at least 48 mm wide, not sticky tape or masking tape. For heavy cartons, add a second centre strip. Self-seal mailing boxes need no tape at all.

Step 7: Check the postage before you commit

How cubic weight works

Australia Post and most couriers charge on whichever is greater: the actual weight or the cubic weight. Cubic weight reflects how much space your parcel takes up in the truck.

Cubic weight (kg) = length (m) × width (m) × height (m) × 250

Australia Post and Aramex both use a conversion factor of 250 kg per cubic metre. Other couriers and freight services can use different factors, so check your carrier's terms.

See the difference box size makes

Carriers measure the external size of the finished parcel, so the real figure will be slightly higher.

Imagine a 1.5 kg product. In the A4 mailer you are charged on about 1.9 kg; in the A3 mailer, about 4.6 kg. That is the same product costing more than double to post, simply because of the box. Under non-contract accounts such as MyPost Business, Australia Post says it generally does not apply cubic charging to parcels under 1 kg actual weight, but it may still do so for large or irregular parcels. For more ways to cut costs, see DIM Weight Reduction & Shipment Consolidation.

Know the carrier limits

For domestic parcels, Australia Post's limits are a maximum of 22 kg, 105 cm in length and 0.25 cubic metres. Aramex carries up to 25 kg actual or 40 kg cubic weight, with parcels up to 2 m long. Anything larger goes by courier or freight. Check our Australia Post Packaging Requirements guide too.

Use boxes that fit prepaid satchels

Australia Post prepaid satchels are priced by size, not cubic weight, for items up to 5 kg. Pop a small mailing box inside a satchel and you get box-level protection at satchel pricing. Browse our boxes that fit AusPost prepaid satchels or read Which Box Fits an AusPost Prepaid Satchel?

Bonus: presentation and branding

Your box is often the first physical contact a customer has with your brand. A neat, well-fitted mailer makes a cheap product feel considered and an expensive one feel worth the price.

• Brown kraft – natural, eco look, lowest cost.
• White – clean and modern; a great base for stickers and stamps.
• Coloured mailing boxes – instant brand colour without custom printing. Standard colour boxes are brown inside; our "Full" colour boxes are coloured inside and out.
• Custom printed – the ultimate brand experience once your volumes justify it. See Custom Printed Boxes: MOQs, Lead Times and Costs.

On a budget? Tissue paper, a branded sticker and a handwritten note go a long way. Ideas in How to Create a Memorable Unboxing Experience on a Budget.

Bonus: sustainability

Cardboard is one of the easiest packaging materials for Australians to recycle. Paper and paperboard make up more than half of all packaging placed on the Australian market, and according to the Australian Packaging Covenant Organisation (APCO) around two-thirds of it was recovered in 2023–24. The National Packaging Targets, which aim for packaging to be reusable, recyclable or compostable, continue beyond their original 2025 deadline, and national packaging regulation is being reformed.

Practical ways to make your packaging greener:

• Right-size – the smallest box that protects the product uses less board, less void fill and less fuel.
• Choose certified, recyclable board – our boxes are FSC certified and 100% recyclable.
• Go plastic-free inside – paper void fill and paper tape keep the whole parcel in the kerbside recycling bin.
• Buy local – our Australian-made shipping cartons travel fewer kilometres to reach you.
• Tell customers – a short "please recycle this box" note helps the box reach the right bin.

Browse our full eco friendly packaging range.

Test before you buy in bulk

Professional packaging labs use ISTA drop-test procedures, but you can run a simple version yourself:

1. Pack three to five real orders exactly as you would for a customer.
2. Drop each one from about waist height (roughly 75–90 cm) onto a hard floor – on the base, on a corner, on the longest edge and on a side.
3. Shake each box and listen for movement.
4. Open them and check the product and the box.

If anything breaks or the box splits, step up the board, add cushioning or move to a closer-fitting size, then test again. A few dollars of samples now is far cheaper than a pallet of the wrong boxes.

Quick picks by product type

Common mistakes to avoid

• Reusing old boxes for heavy goods – every trip weakens the board.
• Confusing internal and external sizes – fit by internal, budget postage by external.
• Choosing one "universal" big box – it saves thinking but costs postage and void fill on every order.
• Using a mailer for weight it wasn't designed for – thin E-flute is great for presentation, not for 10 kg of tools.
• Skimping on tape – one strip down the middle isn't enough for a heavy carton.
• Ignoring the satchel option – a small box inside a prepaid satchel can be the cheapest way to post.
• Not testing – a five-minute drop test can save a season of breakages.

Need a size we don't stock?

Frequently asked questions

What size box do I need for shipping?

Measure your product at its widest points, add clearance on every side (none for robust items, 15–25 mm for presentation, 50 mm+ for fragile goods), then choose the smallest box whose internal dimensions are equal to or slightly bigger than the result.

What is the difference between a mailing box and a shipping carton?

A mailing box is a one-piece die-cut box with a built-in lid, designed for direct-to-customer orders and great presentation. A shipping carton (RSC) has four taped flaps, is stronger per dollar and stacks well, making it better for heavy, bulky and wholesale shipments.

When should I use a double-wall box?

Generally once the packed weight goes above about 10 kg, or when the goods are fragile, high value, or will be stacked on pallets or in storage.

Does Australia Post charge by box size?

Yes, for parcels in your own packaging Australia Post charges on the greater of actual weight and cubic weight (length × width × height in metres × 250). Prepaid satchels are the exception: they are priced by satchel size for items up to 5 kg.

What is the biggest box I can send with Australia Post?

Within Australia, parcels can be up to 22 kg, 105 cm long and 0.25 cubic metres in volume.

Are cardboard boxes recyclable in Australia?

Yes. Clean cardboard boxes go in the kerbside recycling bin across Australia. Remove plastic void fill first; paper tape and paper fill can stay in.

Can I get samples before buying in bulk?

Yes – order a small pack or request a sample, pack a few real orders and run the drop test above before committing to a bulk order.

Sources and further reading