Should I cancel this order?

[catnipalley]

I have a customer who has sent me a snotty message this morning.  The item he bought ended on 7/23, and when he didn't pay within the time stated in the auction, I filed an unpaid item case.  He finally paid on the 30th, but sent a snippy message this morning about how "it's always the ones who overcharge for postage who are too quick to file a case."  Am I in my rights to send his money back and cancel his order?  I can just feel negative feedback coming on.