Credit Limit

Maybe a soft-limit, hard-limit approach would be a good idea?
An email could be sent asking for a payment before running out?
(Say by 80% of the limit?)

Or setup authorized CC automated billing for ever $100 in extras?

Just thinking . . .

The threshold could be raised over time based on succesful transactions.
Some companies have CC thresholds, and you will need to bill in increments anyway.