In Marketpress, where do you pull the originating address from when calculating for USPS shipping?

I have the USPS calculating all set up, but realized I set up the account with them with my mailing address, not the address the packages will be shipping from. I went back and created a new account with them with the correct originating address but it's not impacting the prices. Do you pull from when I set up my account with you? How can I change that address that you're using as the originating address? Thanks!