Do cancelled orders really count on pay-as-you-go?

Yes, and it is stated before you choose the plan rather than discovered afterwards — you have to tick an acknowledgement that says so. The reasoning is that an order consumed the work whether or not the kitchen finished it: the messages went out, the ticket printed, the customer was served a page. Your billing page lists every counted day and each date links to your own Orders board for that day with cancelled orders visible, so you can count for yourself.