> charge you an amount whenever they wanted, without user authorisation?

It's a simple matter of user communication: you make it clear from the start that the billing will be unpredictable, and potentially provide a ceiling for monthly bills to let the user stop if it goes out of hand.

I follow per creation billing creators and it's fine. Amazon also offers an option to auto buy new volumes of a series.

The customer not knowing in advance how much they'll be billed isn't common, but it's not complex in itself.