Don't allow an uninvoiced ticket to be resolved without a reason.
If you resolve a ticket they go away, if someone resolves a ticket that should have been invoiced we lose that money. if RS forced you to supply a reason and that was added as a private comment to the email (or even added to the communication log as (ticket #4456 marked resolved with no invoice. Reason: "this reason")
Juan Roman commented
and or allow users outside of the admin to see pending ticket charges listings as a main report to catch this resolved tickets