Gave buyer $20 refund when I didn't have to and this is how they thank me (plus negative feedback)

Sold a card a while back and turns out it's delayed by USPS (with tracking). They gave me immediate negative feedback right after Christmas.

They didn't get it in time for Christmas so I felt bad and gave the refund even though I didn't really need to yet. Told them that I did it and said "Happy Holidays" and this is how they responded.

Like who tf behaves like this?