Hi,
There are multiple ways you can accomplish this (depending on your preference) but at the moment I am able to come up with the following suggestion. You can use the onInventoryItemUpdated( ) event handler to call the emailing function . The onInventoryItemUpdated( ) event handler runs when information for an inventory item is updated in a store. Keep in mind that this is part of the wix-stores-backend module.
Best regards,
Edward Corvid Team