I have pendants, in silver, gold, and platinum. When a user buys one or more of the variants, I want to mark the quantity of all three variants to zero, expiring the pattern. What is the best way to do this? I was thinking some sort of custom app that could listen for a purchase event? I was also thinking I could generate an email on purchase, and have a service account listen to that inbox, and trigger an expiration through the API. But that seems klugey.