Interesting question! I would assume that some invoice must be generated, but it can be in electronic form (e-mail). Getting this right internationally is still a major problem as far as I'm aware, but would love to know if there is a (cheap enough) service that solves it...

Users are charged only once every 24 hours aggregating their usage of the previous period. This way the number of charges/fees/invoices are limited to a relatively small number.