Writing a simple Point of Sale app with WordPress

Hi .. I’m thinking of extending the WordPress-based website that I’ve done for my daughter’s shop by creating a POS (Point of Sale) application which would use some password protected pages on the site. What I’m looking to do is to allow the shop assistants to click on one or more options on a home (sell an item, register an expense, log an incoming item for sale and a few others. So the home page would then go to a number of other pages each with it’s own form, the data from which would be logged in a database and then extracted for later processing in the accounting system. At the moment, everything is written down in a paper book and then inputted into the accounting system manually.

Question is what do I need to do this. Gravity forms looks impressive but I’m not sure from the documentation exactly how I can handle the back end of things i.e. moving data between the forms and database. With Gravity forms, do I need an add-on or use the API or something else ? I’m finding it hard to get my head around exactly how Gravity forms interacts with the application logic.

Or should I do it another way ?

Can anyone advice please ?

Thanks
Ron