Just remember that if you don't have enough weight to warrant using a flat rate box then it isn't a savings. You can ship a lot in the flat rate envelope for $4.95. I saw a gal mail one of these envelopes that I thought was supposed to stay flat, but hers was almost a round ball! So I asked the worker and they said they let them go through as long as it can be sealed by the peel n stick thing on them.
I know if you don't have a scale at home it is a lot of trouble, but it can save you a lot of money to send regular priority, Especially if you are shipping within your state or not too far away...regular priority is figured by zone.