Best practice for canceling an MVC Order

Question

I have a store that will not require logging in. A customer can order as a guest, and fill in their shipping details, etc.

(Payment is Paypal. ) If a customer cancels Paypal and disconnects, I would remove their data, but exactly how would I know they canceled? What's a good way to know?

One idea I have is to fetch some kind of token from Paypal if they paid. If they didnt pay, then I could remove their data. But what if they are still connected, and they are just being undecided for a half hour?

I ask because I was using session data, where I only persisted if they paid. But MVC is good at persisting data, and now I don't know whats best.

To persist or not to persist...that is the question!