I've had two parking tickets in my life - one a few weeks ago and one almost a decade ago - Ive still got both receipts, on the off chance something like this happens.

I assume that it's not a receipt like from a store, but a governmental "receipt" - a document proving payment of fine, which is something I'd definitely save on purpose.

(I'm not from the US, I have no idea how yours look - in Europe we have that on an A4 paper)