Best Pick
eBay
300 9x12 Poly Mailer Self Sealing Shipping Envelopes Waterproof Packing Bags
9″ × 12″ × 0″
+$12.40 USPS Ground Advantage · 3 lb billed
$0.07
each
pack of 300 · $21.00
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Cheapest 12×9×3 padded mailer
Compare real eBay prices for 12×9×3 shipping boxes with DIM-weight-aware shipping estimates.
Live eBay listings — sorted by cost per useful box
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eBayeBay Brand Central
3” eBay Sticker – Red, Blue, Green and Yellow — 500 Stickers
12″ × 9″ × 3″
+$12.40 USPS Ground Advantage · 3 lb billed
$0.09
each
pack of 500 · $45.00
eBay
100 Pack 9x12 Inches White Poly Mailers Self Seal Closure 2.5 MIL Thick heavy Du
9″ × 12″ × 0″
+$12.40 USPS Ground Advantage · 3 lb billed
$0.09
each
pack of 100 · $9.00
eBay
9x12 Clear Poly Mailer Bags Self Seal with Warning – 100 Pack Shipping Bags
9″ × 12″ × 0″
+$12.40 USPS Ground Advantage · 3 lb billed
$0.13
each
pack of 100 · $13.00
eBay
Poly Mailers 9x12 White 200 Pack Waterproof Tear-Proof Self-Seal Shipping Bags
9″ × 12″ × 0″
+$12.40 USPS Ground Advantage · 3 lb billed
$0.20
each
pack of 200 · $40.00
eBay
Poly Mailers Heavy Duty Shipping Bags | Self Seal | 9x12 | 100 Pack
9″ × 12″ × 0″
+$12.40 USPS Ground Advantage · 3 lb billed
$0.14
each
pack of 100 · $14.00
Showing 6 of 32 real listings. Want all of them with custom quantity and weight? Search now →
What fits in a 12×9×3 box
A 12 × 9 × 3 inch box holds 324 cubic inches (0.19 cu ft) of interior space, putting it in the medium-category. It's a popular choice for shipping items like hardcover books, shoes, small electronics — things that fit comfortably within those dimensions while leaving room for protective padding. Bulk packs of 25, 50, and 100 are the common listing sizes, with per-unit pricing typically between $0.60 and $2.00 depending on quantity and seller.
Shipping math for this size
Shipping cost for this size depends heavily on dimensional weight. For a 12×9×3 box, UPS and FedEx Ground compute a DIM weight of 3 lbs using their 139 divisor, while USPS Ground Advantage uses a 166 divisor, giving 2 lbs. Carriers bill whichever is greater — your actual weight or the DIM weight. USPS's higher divisor makes it roughly 1 lbs cheaper than UPS/FedEx for this size when your item is light, which is why our search results factor in DIM-weight automatically instead of just showing sticker prices.
How to pick the right pack size
When buying a 12×9×3 box, the biggest cost lever isn't which retailer you pick — it's which pack size you commit to. A 100-pack typically costs 2-3× more than a 25-pack but brings the per-unit cost down by 40-60%. Our ranking is based on *cost per useful box*, which gives bonus boxes partial credit — so a 100-pack can still win for single-box needs if the per-unit price is low enough, but a 25-pack often ranks higher when you only need a few. Every listing below links to a real eBay product page; we don't show placeholder results.
Search with your own dimensions
Pre-filled with 12×9×3. Adjust weight, quantity, or size as needed — we'll re-rank for your exact scenario.