How do I set up inventory tracking for my PayPal button?
You can set up inventory tracking for your PayPal payment button and automatically receive email alerts when your inventory runs low. For details, see Track inventory, profit, and loss on the PayPal Developer Portal.
Note: Inventory and profit and loss tracking don't apply to Donate buttons.
Receive payment notifications
BMSetInventory API Operation (NVP)
BMGetInventory API Operation (SOAP)
BMGetInventory API Operation (NVP)
PayPal Payments Standard Button Manager API Overview