If an item shipped by an American seller under GSP is lost in the mail, the buyer gets two refunds:
1) from the seller for the full purchase price
2) from Pitney-Bowes for whatever shipping and "import fees" were paid through PayPal.
In case of returns, it is a bit more complicated. One would have to check each individual case with eBay customer support, depending on the exact circumstances why the item is returned. In case of damage in transit for example, many buyers have been told to simply discard the damaged item and received both refunds.