I agree that flat rate is the best way to go, but there is a little thing to remember. I was on a long line for a while because they didn't have flat rate boxes out. The line was even longer after me and I didn't want to make the line again. I asked if I could pay for the stamp there too, and they wouldn't do it. They just gave me the box. I asked why, and they said that there is a weight restriction. I am not sure what the weight restriction is, but I guess if you are mailing books and magazines, be aware. They say that if it fits in the box it goes, but is not 100% true. If it is over certain weight, it doesn't go flat rate.
They have a table at the post office with the weight retrictions.