I occasionally sell old books via the Amazon Marketplace. Shipping is normally media mail (although I do use first class for single CDs). Media mail has always been a pain, because you need to take it to the counter.
I just learned you can print and pay for media mail postage at Paypal using their “PayPal MultiOrder Shipping” (note that their application doesn’t recognize SeaMonkey as being Mozilla, but you can still get it to work, and requires enabling popups). Should make life a lot easier.