eBay dispute question

I’ve had issues like many with USPS during the Holiday and am unsure on best way to do things. 2 scenarios.

1.) a seller gave me a refund for a card, but i just got it today (was supposed to be here December 7th) - there’s no return to sender option on eBay. Any advice?

2.) I sold a card that got mangled by USPS and buyer rightfully wants a refund. With that said, I’d like a refund too as I’m now out the money and the card thanks to USPS. What do I need to do?