If a customer ever changes their billing rate, you will need to keep track of when it was changed and what their previous rates were, otherwise you'll have no audit trail and no way to bill customers correctly.

One approach would be to have a billing_period (account_id, billing_rate_id, start_date, end_date) table. Another would be to store the billing_rate_id in your account_credit table so each usage can be tied to the billing rate the customer should be charged for it.

I also wouldn't keep a running_balance column. I forget which rule of normalisation it is because it's been so long, but you shouldn't normally store a value that you can compute from other data.