This is also a game changer feature for my business - in particular, the ability to add quantities for Add-ons.
ie - I have a white label SaaS subscription that allows me to sell 50 sub-licenses. I would like to sell them priced as monthly recurring subscriptions, so when the 50th one is subscribed to, the add-on automatically becomes no longer available to purchase.
However, if all 50 are being used, but someone happens to cancel their subscription to one of them, making the total used 49, then that add-on should become active again on the Pabbly checkout page, available for purchase.
It would be great if we could choose whether or not to show the "sold out" product on the checkout page - so if they're all in use, we can choose if (a) it still shows that add-on name with a note next to it saying it's "sold out", or (b) the add-on gets removed completely from the checkout page so people don't know that it was ever even an option to purchase.
The functionality should of course be tied to the Customer Portal, so that if someone upgrades to purchase the add-on within the Customer Portal, it also has the same effect of no longer being available on the Pabbly checkout page, etc.