Product, Orders, and Invoices

Hey Admin,

I apologize if you get this twice. I submitted this question yesterday, but it is saying I have no posts. 

I am looking to have my product quantity automatically deduct when I submit either an invoice or and order (I'm not sure which one this can be done with). I am assuming it can be done when an invoice is paid or an order is completed. I have been looking through Academy to figure this out, but haven't been able to find it. Please help.

Thank you!