Backordered Email Not Clearing After Customer Buys Item.

We use a POS system that was developed using Filemaker. It uses the REST Api to communicate with cs-cart.
Here is the scenario
There is a product that is out of stock. On our website, people will add their emails to be alerted when the product is back in stock.
The product goes back in stock. As normal, cs-cart sends out the emails to those who wanted to know when that item is back in stock.
If a person, who was notified by email, purchases the product online, Again, as normal, their email gets removed from the subscriber list for that product.
The problem we are running into is if someone comes into the store to purchase that "now in stock" item, (The POS system is used for this transaction) their name does not get removed from the subscriber list for that product.
Now if it's an item that consistently runs out of stock, someone who purchased that item in our retail store will always get an email when it is back in stock, since their email has not been removed. You can see how annoying this can get for certain customers.
I am assuming that a REST delete method would need to be executed from our POS system?