U.S. Postal Service flat rate envelopes and boxes can be a big money saver under the right circumstances. But you have to be careful. That's because there are now seven domestic USPS flat rate size and rate combinations, and potential cost savings depend on a fairly complicated comparison of package volume, weight and distance.
Use a Spreadsheet to Compare Rates
The easiest way to manage this problem is to create a spreadsheet that presents all the rates in one document. There is a flat rate mail comparison chart that I use and you are welcome to download a copy by clicking here.
How to Shop Flat-rate Shipping
Step 1 - Pick a flat rate envelope or box that will contain your shipment, loosely. Pack the items and then weigh the package and contents.
Step 2 - Find the priority mail charge by weight and zone on the spreadsheet, and compare this to the flat rate for the smallest envelope or box that works for your shipment.
For example, if a medium flat rate box (FRB1) that is 11" x 8.5" x 5.5" is large enough for your shipment, and if the ship weight is 5 pounds with a zone 3 delivery address, you can quickly compare the $8.15 priority mail rate to the $10.35 flat rate. In this case, you would waste money by shipping at the flat rate.
But, if the delivery zone for that 5-pound package was instead zone 8, the priority mail rate increases to $17.50. So, using a flat rate box in this case would cost much less.
You can also add UPS or FedEx ground rates or USPS first class mail, media rate and parcel post rates if you anticipate using these other delivery options in place of USPS priority mail delivery.
The extensive range of U.S. Postal Service flat rate envelopes and boxes can be a huge money saver if you ship in volume, but it also pays to shop flat rate shipping against regular delivery costs on a shipment-by-shipment basis.