I believe that flat rate is the same to all US Addresses. A first class letter is the same but if you send a small package it can be different depending on the zip code, I think! If it is priority but not flat rate there is a difference. I can remember that some tax returns we sent out ever a different amount for some zip codes, even when they weighed the same. That was for large envelopes that were over 13 oz (but didn't fit in the flat rate envelope)