We're using the API to create multiple event registrations via a single form, then send the user to Quickbooks Payments after submission for those who wish to pay a deposit online. The bookkeeper sends the back office staff a weekly QB report of all payments received that week (online, checks, cash, etc), which they then enter into Civi manually.
Is there an "invoice number" or some other unifying ID we could use to help staff easily find the appropriate payments from the bookkeeper's report and create them on the correct Civi registrations? I see a Cart ID in the civicrm_participants table, but the cart functionality doesn't have an API, so I can't insert in a safe way. Should I just go with the Source field, or a custom field?