I would get rid of the billing_rate table or at least copy its contents into billing_account - nothing good can come of changing that reference! Also consider

I would get rid of the billing_rate table or at least copy its contents into billing_account - nothing good can come of changing that reference! Also consider how you will build invoicing on top of this - i.e. A monthly PDF that covers (for a period) prepayments made, services rendered, balance remaining. It seems like your billing model makes that quite easy but bear in mind you have to be able to sum that up on a particular date, and for that summary to be immutable.