Thank you all. . .it really seems dodgy to me, and alarming, to have received a collections call from a guy two months after the check was cashed, by them. He also claimed he wasn't a collector, which is just hogwash.

*) Is he correct that I am required to provide proof of payment? When it's minimally their sloppiness?

*) If I refuse to provide proof of payment, can he continue to pursue the matter?

*) Is email for all of this a good idea, or should I start with the certified mail/return receipt business?

Sorry if I appear to be over-reacting, but I've just never been in this position before. And have been warned by so much about trying to work with these people. I had so hoped that I could just pay and this would go away: it was a legitimate debt, but had become overdue due to no fault of my own.