The U.S. Postal Service tries to portray their Priority Flat Rate Shipping service as a great deal. This is not always the case. Yes, it is easier just to throw something in a Flat Rate box and be done with it, but a tiny bit of research could save you big bucks, while the package still arrives at its destination in the same amount of time!
Knowing the weight of the package you are shipping is the key to getting the best deal. Figure the approximate weight of the entire package - item(s) + packing materials + the box itself. Note: the average sized Flat Rate box (empty) is approximately 7 ounces.
Know your limits...
First of all, let me tell you that the Small (8-5/8" x 5-3/8" x 1-5/8") Flat Rate box is the same price ($4.95) as ANY size regular Priority box, 1 pound and under. So if your package weighs 1 pound or less, regular Priority is the better deal since you can use any size box within the Priority Mail guidelines ("The maximum size is 108 inches or less in combined length and distance around the thickest part"). Note: You do not have to use a Priority box for a regular Priority package.
If you would need a Medium (11-7/8" x 3-3/8" x 13-5/8" or 11" x 8.5" x 5.5") Flat Rate box for a package that is 2 pounds or less, regular Priority will be the cheaper way to go ($8.70 Priority vs. $10.35 Flat Rate), and again you can use any of the size boxes within the Priority Mail guidelines.
If a Large (12" x 12" x 6") Flat Rate box would be required, regular Priority Mail is going to be your best deal if you package weighs 3 pounds or under...$11.95 (Priority) vs. $13.95 (Flat Rate).
To sum it up, the Flat Rate boxes are Small - $4.95, Medium - $10.35, Large - $13.95. Priority Mail (any size box that meets the guidelines) is $4.95 (1 pound or less), $8.70 (2 pounds or less), $11.95 (3 pounds or less), $14.70 (4 pounds or less)...see http://postcalc.usps.gov/ for prices on heavier packages.
Take your package to the post office and be happy with the money you've saved! Share these tips with friends!
Once you figure out the exact weight of your package, if you find that it will be cheaper to go the other route (from Flat Rate to Priority or vice versa), remember you can always repackage it. If you ship out packages fairly often, grab a few of each kind of box when you are at the post office to keep on hand for your next mailing.
These prices are based on Domestic shipping and depend on distance of destination, etc., but these rules should be applicable to most destinations.