USPS Flat Rate Boxes — Sizes, Prices, and When They Save Money (Live)
Prices last refreshed May 3, 2026. Source: live USPS rates via Shippo's discounted commercial rate API.
What flat rate boxes are
USPS Priority Mail Flat Rate boxes and envelopes have one price regardless of weight or destination (within the U.S. and to military APO/FPO/DPO addresses). Pack the box, ship it anywhere — same price up to the 70 lb weight limit.
The boxes themselves are free. You can order them from USPS.com or pick them up at any post office.
Live USPS flat rate box prices
Discounted commercial rates available through shipping software like Stamps.com. Retail counter prices are typically 10–15% higher — see USPS Notice 123 for retail.
| Box | Dimensions | Priority Mail | Priority Mail Express |
|---|---|---|---|
| Small Flat Rate Box | 8.625×5.375×1.625" | $10.59 | — |
| Medium Flat Rate Box (side-loading) | 13.625×11.875×3.375" | $19.98 | — |
| Medium Flat Rate Box (top-loading) | 11×8.5×5.5" | $19.98 | — |
| Large Flat Rate Box | 12×12×5.5" | $27.76 | — |
Live USPS flat rate envelope prices
| Envelope | Dimensions | Priority Mail | Priority Mail Express |
|---|---|---|---|
| Small Flat Rate Envelope | 10 × 6" | $9.62 | — |
| Flat Rate Envelope | 12½ × 9½" | $9.62 | $31.11 |
| Legal Flat Rate Envelope | 15 × 9½" | $10.91 | $31.43 |
| Padded Flat Rate Envelope | 12½ × 9½" | $11.02 | $31.70 |
When does flat rate beat by-weight pricing?
Flat rate is a win when your shipment is heavy for its size and going a long distance. Two scenarios:
- Heavy items in a small box. A 15 lb box of books or tools that fits in the Small Flat Rate Box ships for the small flat rate price — by-weight Priority Mail for the same weight going coast-to-coast would cost more than double.
- Cross-country shipments at any weight above ~3 lb. Priority Mail by-weight pricing increases with USPS Zone (1 = nearby, 8 = far). Flat rate doesn't care about zone, so it gets relatively cheaper the farther you ship.
Flat rate loses when you're shipping a light item a short distance — by-weight Priority Mail for a 1 lb shipment to Zone 2 will easily beat any flat rate price.
Compare against by-weight Priority Mail rates
Use the shipping calculator on our home page to compare flat rate against by-weight Priority Mail (and against UPS and FedEx Ground) for your exact origin, destination, and weight. The calculator surfaces both flat rate and by-weight options so you can see when each wins.
How to actually book flat rate at these prices
The prices in the tables above are discounted commercial rates. To get them you book through shipping software, not at the post office counter:
- Stamps.com — code MAIL100 for $100 in postage credits
- Other shipping software (ShippingEasy, Shippo, ShipStation) offers the same USPS commercial rates — see our discounts page for a full list
At the post office counter you'll pay USPS retail prices, which are roughly 10–15% higher than the commercial rates above. Same boxes, same delivery — different price because USPS charges shipping software at wholesale.
Methodology
Prices fetched from Shippo's discounted commercial rate API for each USPS flat rate
template (USPS_SmallFlatRateBox, USPS_MediumFlatRateBox1, etc.). Stored in our
flat-rate-prices Supabase table and refreshed monthly via Netlify cron.
For the related flat rate
box sizes and dimensions guide, the same data source feeds both pages — they
can never disagree.