Product relationship with 'transactions' and 'users'

Hi

I have been reading about but just struggling on the best approach to achieve a relationship between multiple forms and database storage.

So I basically want a 'Product' to store a 'transaction' and relate this to a 'user'. I assume I need to make a one to many relationship for a product to store multiple transactions. I have read the docs and think I can do this part.

Currently I have 2 separate forms ('Product' and 'Transaction') and both save to a custom table. What is the best approach to combine these forms into one? so the user on the front end can fill in a 2 part form essentially and have this store all the data into 2 separate tables but linked by the product id?

Is there a way to extend the 'register_meta_boxes' method to add in extra form field programmatically? or can i combine the forms using the shortcode?

I hope that I am making sense.

Thanks