My experience has been is that if your item is less than 13 oz., first class is the way to go, cheap and fast. Heavier items are worth the flat rate. I buy the clasp envelopes in multiple sizes from the office stores, and use them for shipping things like books, patterns, magazines and fabric. I actually shipped two yards of fabric to Canada for less than four dollars(first class) in one of those! I forgot to say that I remove the metal clasp and reinforce the seams with tape.