Paypal lets you create purchase buttons, and one option is to make the purchase actually trigger an autopay - they call it a Subscription/Subscribe button. They also let you enter inventory for a given item.
If you enter 1 in Inventory for a Subscribe button, does Paypal:
A) Treat that as 1 item in stock, and after 1 month the subscription will run out
B) Treat that as 1 Subscription in stock, and after one person purchases it, it will be out of stock (but the Subscription that 1 customer purchased goes on indefinitely)
I assume B, but perhaps others have found otherwise.