Most generally in a situation like that, the charity would have the calendars (order info and ads) on the charity website, the person would pay the charity direct, and the charity places the orders and either has it sent to buyer, or has all sent to charity for pick-up (if it's local) ...

Calendars are usually a one-time late year item for the coming new year, and so it would make sense for the charity to collect orders late in the year for the coming year, then order all at once ... with some extras for late donors or individual fundraisers.