You can set up inventory tracking for your PayPal payment button and automatically receive email alerts when your inventory runs low. To set up inventory tracking for PayPal buttons, go to the button creation page and choose between the available tracking methods. For detailed steps, visit Track inventory, profit, and loss on our Developer Portal.
Note: Inventory, profit, and loss tracking is not applicable for Donate buttons.
See also:
Receive payment notifications
BMGetInventory Operation (NVP)
BMSetInventory Operation (NVP)