Like I said previously, individual patrons can set their own monthly maximums. But really the only thing that prohibits the creator from doing that is that it will piss off their patrons, and if the patrons don't feel that they're getting their money's worth, they can cancel their patronage. Which they will do.
And that's totally sufficient.
It helps to get over the emotional hump if you realize that it's not actually any different than invoicing somebody through PayPal. If somebody hires you to landscape their yard, and you send them an itemized invoice through PayPal's invoice system, PayPal isn't going to check to make sure that you actually edged the lawn and planted the begonias like agreed.