"Cash" as in currency? WOW.

You need to find your receipt marked paid. And like every other receipt that you pay with cash that isn't for a trivial purchase like lunch, I would make sure you still know where it is 30 years from now.

If you don't have the receipt, you may have to pay for the service. I doubt there is any legal basis for them to try to collect $600 for collection fees. What does your service contract with them say?