If you're thinking, "but how many works does a creator ship each month?": no, that's the whole point of by-works. It can be ANY number. The creator makes no particular commitment to their patrons as to how many creations they're going to ship. They may ship NO creations in a month, so in that month their patrons are charged nothing. Or they may ship one thing. Or 10 things. Or a 100 things.
(My best month was 10 things.)
You might reasonably be thinking, "migod, what protects patrons from getting socked with an enormous bill because a creator ships many more things then they expected?"