Subscription verification when selecting a product

Hi,

I have just installed opencart 2.x to create an online payment portal. The trick is it is meant to be for payments that require verification (user selects product, user enters subscription number, subscription number is pushed to an API for verification/validation returning subscribers name, user confirms and product is added to shopping cart). My first thought was maybe to use vqmod but first wanted to consult incase someone has had a similar experience or knows a better method.

Thanks