I am going to assume that the rules for the refund are this -- for an over-charge of $x:
1. Customer gets a min of $1 + x
2. If the overage is between $0.10 and $0.50, the customer gets a rebate of 10 times the overage plus the overage, for a total of 11 times x.
3. If the overage is $0.50 or more, then the custmer gets $5 plus x.
If this is correct, then you have: