If you're running low on stocks with your Add-ons and don't want customer to be able to order these add-ons if they're out of stock then follow the steps below to make it disappear as one of the option when add-ons are out of stock. 

First, add this product as a website product under " Product Type " then fill up all the details like Product name, Product ID, description, pricing and category. 

Second, scroll down to the very bottom where you see the "Advanced setting", click on it. Now select Yes on the "Manage Inventory" and a new option will appear called "Units in Inventory" now place a numerical value on how many you have on stocks then hit "Save" at the top right. 

Lastly, while on your on that page. Change the product type to "Website Add-on" and hit Save. It'll be changed to an add-on with inventory feature applied to it.