Use CB Activity for Transaction Log

We've implemented PayPal IPN (Instant Payment Notification) and receive IPN payment notices about our uses. I pass the Joomal user ID to PayPal in their custom field, and that's how I tie it to a user on the return IPN message.

Currently, payment is for just one event, but in the future, we may end up having multiple transactions to record for a user. Transactions could be a payment or a refund.

Would CB Activity with an Activity and/or Comment field be a good solution for this? I'm even thinking the value of the Activity/Comment could be a JSON string with all the details, then displayed via CB Code Field to parse that data.

Does this fit within the scope of the add-ons, or do you have other suggestions?

I mean sure it could do that, but I really don't recommend it. The recent/all activity page will end up displaying that activity. A comment stream would probably be more secure since comments don't have a recent/all page displaying all comments. Whatever you're using to implement the payment system or IPN handling ideally should be responsible for all of this with its own database table to keep track of payments. I recommend CBSubs which already has all of this functionality.