Updated July 2, 2026
See the numbers for your package
Punch in your package's weight and size to see whether flat rate or weight-based pricing wins this time.
Short answer
Flat rate wins when you are shipping dense or heavy items to distant zones; weight-based wins when packages are light or going to nearby zones. A 15-lb item shipping to zone 7 via USPS Priority Mail weight-based would cost $35 to $45, versus about $22 in a USPS large flat rate box. The crossover point depends on your package's weight and destination, which is why running both numbers on each shipment is worth the 30 seconds it takes.
Payment table
| Service | Flat rate option | Max weight | Approx cost |
|---|---|---|---|
| USPS | Flat Rate Envelope | 70 lbs | ~$10 |
| USPS | Small Flat Rate Box | 70 lbs | ~$10 |
| USPS | Medium Flat Rate Box | 70 lbs | ~$17 |
| USPS | Large Flat Rate Box | 70 lbs | ~$22 |
| FedEx | One Rate (various sizes) | 50 lbs | Varies by size |
When flat rate beats weight-based
A 15-lb item shipping to zone 7 via USPS Priority Mail weight-based would cost $35 to $45. The same item in a USPS large flat rate box costs around $22. No negotiation required, no zone math, just the box price.
When weight-based beats flat rate
A 2-lb item going to zone 2 might cost $8 weight-based via USPS Priority Mail but $17 if you reached for a medium flat rate box. Flat rate boxes are not free: you pay for the right to ignore weight, and that premium only pays off for heavy, distant shipments.
Things to know
Calculate the weight-based rate for your package (actual or DIM weight, whichever is higher, multiplied by the zone rate). Compare that to the flat rate option for the smallest box that fits. If flat rate comes in lower, use it. If weight-based comes in lower, ship normally. The rate estimator shows a weight-based modeled figure in one view.
Run your own numbers
Once you have picked a box, check both pricing paths before you commit to one.