My idea is that the customer scans a key tag which carries a barcode each time they make a purchase. The database was just designed to collect entry data, as in when the customer entered the bar. I think it should be simple enough to add another field to that table to collect a value amount, like 30p or 10%, and then we should be able to query/report that amount as it cumulates and redeem it periodically as and when the customer wants to spend it.
I'll copy everything first so my ham fistedness can't do any harm and start having a look.