I'm thinking of putting up a couple things too and I noticed when I went to the Post Office that prices have gone up again. That tiny tiny box that was 4.95 to ship, went up to 5.20 and so did those others where 'If it fits, it ships'.
First off, they are going to go by how much it weighs and then the zip code. You can go to the USPS site and put in the zip code, say for New York if you are shipping from West to East and how much you think it will weigh and it'll give you a ballpark price.