I received a great piece of advise on this site when I was new last year: Avoid the post office counter at all costs! Once you figure out how rates work, it is cheaper and easier to do it on your own. For PWE's I have a supply of Non-machineable stamps, additional ounce stamps and forever stamps. For BMWT and boxes it is cheaper and easier to use an on-line service (I use Pirateship.com but there are many others) and print my own labels. I do a lot of Canadian PWE trades and keep a sheet of international first class stamps on hand. The first time I tried to mail cards to Canada (two hockey cards in a greeting card sized enveloope) the clerk wanted me to fill out a customs form. Another clerk attempted to bend a PWE to determine if it was rigid. Be your own postal clerk!
p.s. The Postmaster probably doesn't approve of this but the Priority Flat Rate 12.5 X 9.5 envelopes in the lobby make great cardboard liners for PWE trades. Three simple scissor cuts yield 3 liners per postal envelope.