Re: What about pre-payment?
You get buy pre-paid "credit cards" with a fixed amount of money on them. They are intended for people who need a credit card, but can't get one for credit history reasons, but need the use of one. I don't see why a cloud service wouldn't accept one.
Alternatively, if you use a genuine credit card, as opposed to a debit card, they usually have a credit limit built-in. The maximum financial damage might be higher than you would like, but it won't be unlimited.