Updated June 17, 2026
Compare carrier rates for your package
Enter weight and dimensions for side-by-side ballpark rates from USPS, UPS, and FedEx.
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. The crossover point depends on your package weight and destination, which is why running both numbers on each shipment is worth the 30 seconds it takes.
Flat rate options overview
| Service | Flat rate option | Max weight | Approx cost (2025) |
|---|---|---|---|
| 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.
The simple test
Calculate the weight-based rate for your package (actual or DIM weight, whichever is higher, multiplied by the zone rate). Compare to the flat rate option for the smallest box that fits. If flat rate is lower, use it. If weight-based is lower, ship normally. A rate estimator shows both in one view.
Compare carrier rates for your package
Enter weight and dimensions for side-by-side ballpark rates from USPS, UPS, and FedEx.