Super easy to use. Quickly delivery, very happy with my purchase.
Description
Maximize postage savings while safeguarding small, flat fragile goods with the Boxmore A4 Superflat White Mailing Box B21 (310 x 220 x 16mm). Engineered specifically with a ultra-slim profile under 20mm, this rigid corrugated die-cut box allows you to qualify for Australia Post Large Letter untracked and tracked envelope rates rather than costlier parcel tiers. Combining high crush resistance with a clean, professional white finish, it keeps flat items completely flat and protected against drops without bending during transport.
Key Features
-
Australia Post Large Letter: Ultra-slim 16mm depth strictly stays under the Australia Post 20mm height threshold, slashing fulfillment and postage costs on small dispatches.
-
Rigid Crush-Proof Armor: Strong die-cut corrugated board provides superior edge and face protection compared to standard paper envelopes, preventing flat goods from getting bent or crushed.
-
A4 Surface Profile: Broad 310 x 220mm footprint perfectly fits flat A4 merchandise, small prints, and thin tech items without creasing.
-
Clean White Exterior: Crisp white finish delivers a clean, professional appearance that elevates unboxing and accepts address labels cleanly.
-
Quick Assembly & Tape-Free Fold: Precise die-cut construction folds together quickly with integrated locking tabs for fast fulfillment line assembly.
Product Specifications
| Feature | Details |
| Model / SKU | A4 Superflat Mailing Box B21 |
| Internal Dimensions (L x W x H) | 310mm x 220mm x 16mm |
| Height Specification | Under 20mm (Australia Post Large Letter Compliant) |
| Compatible Size | Fits Flat A4 Documents, Prints & Ultra-Thin Items |
| Box Style | Die-Cut Self-Locking Flat Mailer |
| Board Grade | High-Strength Micro-Flute Corrugated |
| Recyclability | 100% Kerbside Recyclable |
| Color & Finish | Bright White Exterior |
| Guarantee | 90-Day eBPak Guarantee |
Ideal Applications
-
Flat fragile items, enamel pins, jewelry cards, keychains, and small artisan crafts
-
Art prints, greeting cards, certificates, photos, thin books, and A4 stationery
-
Phone cases, screen protectors, memory cards, slim cables, and flat electronic parts
- Lay the flat box printed side down and fold up the side walls.
- Fold the side flaps in so the box holds its shape.
- Slide in your flat item: prints, cards, photos, a scarf or a folded tee.
- Close the lid and tuck the front flap into the front of the box.
- For extra security, seal the front flap with a strip of tape or a branded sticker.
- Free delivery to Sydney & Melbourne metro on orders over $500.
- $50 flat rate to Wollongong areas and $75 to Gosford & Newcastle areas (up to 2 tonne).
- Australia-wide delivery via Australia Post and trusted couriers.
- Free pickup from Minto NSW or Braeside VIC, Mon–Fri 9am–4pm.
- 90-day returns on unopened items. See our refund policy.
Payment & Security
Your payment information is processed securely. We do not store credit card details nor have access to your credit card information.
Sizing
What fits in this box
Inside: 310 x 220 x 16mm (length x width x height)
- A4 documents, folders and books lie flat
- Slim 16mm profile for flat items: prints, cards, photos, scarves and folded tees
- About 1.1 litres of space inside
- Delivered flat, folds into shape in seconds
- Sizes are inside measurements. Allow 5–10mm each side for tissue paper or bubble wrap.
- Not sure? Use the size finder below or call us on (02) 8750 5347.
More superflat mailing boxes
Save $87.75
310 x 220 x 16mm A4 Superflat White Mailing Box B21
Sale priceFrom $16.75
Regular price$104.50
$0.67 each
Mailing Box 180 x 100 x 16mm Superflat Rigid Envelope Mailer B8 White
Sale priceFrom $23.95
$0.24 each
Mailing Box 152 x 132 x 16mm Superflat Rigid Envelope Mailer B1 White
Sale priceFrom $15.95
$0.32 each
Save $6.71
180 x 100 x 16mm Brown Superflat Letter Gauge Mailer B295
Sale priceFrom $23.95
Regular price$30.66
$0.24 each
Size finder
Not sure of the size? Enter your item's dimensions.
Enter your item's measurements on the left, then press Search to see every size we stock that fits.
Nothing in stock fits that size.
Request a custom box quote
Packing essentials
Save $3.41
Heavy Duty Clear Packing Tape 48mm x 75m 52micron A006 x36 Rolls
Sale price$45.36
Regular price$48.77
$1.26 each
Save $10.13
Clear Pallet Wrap 500mm x 285m Hand Stretch Film 25U x4 Rolls
Sale price$62.37
Regular price$72.50
$15.59 each
Wholesale · Bulk packs
Buying in bulk? Pay less per box.
Order 1,000+ mailing boxes, or Australian-made shipping cartons by the pallet, and we'll quote a wholesale price. Trade accounts get invoice terms and a dedicated contact.
Questions about this box
What can I send in a superflat box?
Superflat boxes are about 16mm high inside, made for flat items: art prints, cards, photos, stickers, scarves, folded t-shirts, books and documents. They protect corners better than an envelope and stay thin and light to post.
Is the size inside or outside the box?
All sizes are internal (length x width x height), measured inside the box. The outside is a few millimetres bigger because of the board thickness.
How do I close the box?
Fold the box into shape, slide your item in, then close the lid and tuck the front flap into the front of the box. For extra security, seal the front flap with a strip of tape or a branded sticker.
Can I get this size in other colours?
Most superflat sizes come in white, brown, black, light blue and rose pink. Use the "Same size, other colours" link above the description to see every colour in this size.
Are the boxes delivered flat?
Yes. The boxes are delivered flat in packs and fold into shape in seconds, so they take up very little shelf space.
Can I print my logo on the box?
Yes. We make custom printed boxes from 1,000 units, usually in around 2 weeks. Get a quote. For smaller runs, a printed sticker or stamp on a plain box works well.
What does the pack size mean?
The pack size is how many boxes come in one pack, for example x100 is a pack of 100 boxes. "Number of packs" sets how many packs you get, so x100 with 2 packs is 200 boxes.
Can I return the boxes, or pick up my order?
Yes to both. We offer 90-day returns on unopened items (see our refund policy), and you can choose pickup at checkout and collect from Minto NSW or Braeside VIC, Monday to Friday 9am to 4pm.
Customer Reviews
Overall rating: 5.0 / 5 from 12 reviews.
I’ve used the A4 super flat mailing boxes for the last two years to post out my calendars. They’re a nice product both in terms of protecting the calendars in post, as well as neat presentation. Ive teamed them up with Polycell 300mm x 100m 10mm bubble wrap, a perfect fit on the calendars, and easily cut to length.
Good product, keeps everything safe
Great boxes!
perfect for my needs, met my expectations along with fast delivery and great cost
Great product, fast and friendly service!
Great products, great prices, fast delivery. Preferred packaging supplier!
Love this product.
Always easy to deal with. Good prices and fast shipping.
Exactly as described, great service, fast postage.
A4 super flat white mailing boxes These have saved on postage, they can get posted as a large letter instead of a parcel cost
A4 super flat white mailing boxes These have saved me a fortune in postage, fits my product beautifully and keeps it safe.
Need it today? Pick it up.
Order online and we'll usually have it ready in 15–30 minutes at either warehouse.
NSW
Sydney — Minto
Unit 2, 1 Swaffham Road, Minto NSW 2566
Mon–Fri 9am–4pm · Sat–Sun closed
Get directions
VIC
Melbourne — Braeside
21–23 Endeavour Way, Braeside VIC 3195
Mon–Fri 9am–4pm · Sat–Sun closed
Get directions
Talk to our team
Call us on (02) 8750 5347
Fast delivery Australia-wide
Via Australia Post and trusted couriers
Pickup in Sydney & Melbourne
Minto NSW and Braeside VIC
90-Day Returns
On unopened items