One word of warning for international SELLERS: PayPal does not provide seller protection unless you ship by some method whereby you can get delivery confirmation. That rules out USPS flat-rate envelopes. You would have to go with Express Mail, which costs at least twice as much. Therefore, international sellers may want to consider insuring all packages as that's the only way to protect yourself. USPS does not offer insurance for flat-rate envelopes, so check out
http://www.shipsurance.com. They're what's known as a third-party insurer.
Buyers: PayPal protects YOU all the time.