These shouldn't even exist without a negotiated contract.

I can subscribe to your service for a specific fee on a monthly basis ($20/month say), and take the risk of losing that month's fee if your service or I make mistakes, or I can choose to drop another $20 mid-month, or anything for convenience.

Saying that the computer will "control" the billing and can run haywire tells me that I don't want to be anywhere near your pile of bad incentives.