Under new policies, PayPal will not refund its seller fees if a seller issues a full or partial refund to a customer except under very narrow circumstances (such as payment was sent to you in error, you lost an Ebay case, it was a fraudulent transaction).

I routinely issue partial refunds for combined shipping. My listings have been changed to reflect that I will be holding back a portion of those rebates, so I am warning buyers that they will save money if they "add to cart" rather than making multiple purchases and payments. (If you require immediate payment when using "buy it now" on Ebay, you thwart this plan - so be careful)

This new policy kicked in for one of my accounts in October, and for my second account in November.