Hello Pedro,
I was going to use your approach, but there are multiple submit buttons in our design, and the text boxes aren't directly near the submit button(s) in the markup. In order to match the markup that has already been created and have the submit buttons and repeater appear within the same web part, it makes sense for us to push the entire product page into a web part.
Also, the link you provided (https://docs.kentico.com/display/K8Tutorial/Creating+the+Products+page) is for creating a "Product" page (a standard Kentico page), not a functional e-commerce product page with variants, quantity(ies), and add to cart functionality.