Best Pick
300 9x12 Poly Mailer Self Sealing Shipping Envelopes Waterproof Packing Bags
9″ × 12″ × 0″
+$0.00 Seller Free shipping · 3 lb billed
$0.070
landed per box
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 listings — sorted by landed cost per box
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50 9 x 12 EcoSwift White Poly Mailers Shipping Envelopes Self Seal Bags 1.7 MIL
9″ × 12″ × 0″
+$0.00 Seller Free shipping · 3 lb billed
$0.150
landed per box
pack of 50 · $7.50
eBay Brand Central10" x 12.5" Polyjacket/Polymailer — 400 Envelopes
10″ × 12.5″ × 0″
+$0.00 Seller Free shipping · 3 lb billed
$0.190
landed per box
pack of 400 · $76.00
eBay Brand Central12" x 15" Polyjacket/Polymailer — 400 Envelopes
12″ × 15″ × 0″
+$0.00 Seller Free shipping · 3 lb billed
$0.230
landed per box
pack of 400 · $92.00
250 - 12.5x9.5 SELF SEAL PHOTO MAILERS STAY FLAT
12.5″ × 9.5″ × 0″
+$0.00 Seller Free shipping · 3 lb billed
$0.240
landed per box
pack of 250 · $60.00
50 Pack 9x12 Poly Mailers Self Seal Shipping Envelopes 2.4 Mil Extra Thick Bags
9″ × 12″ × 0″
+$0.00 Seller Free shipping · 3 lb billed
$0.240
landed per box
pack of 50 · $12.00
Showing 6 of 51 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 **landed cost per box** — the pack price plus shipping, divided by the number of boxes. A 100-pack with $10 shipping works out cheaper per box than a 25-pack with the same shipping, because the fixed cost spreads further. Every listing below links to a real product page from eBay, ULINE, or other retailers; 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.