Paper prototype testing on checkout?

I would like to run few iterations of user testing on a checkout re-design. This project is still in the early stage so I would like to take full-advantage of the situation and run testing sessions on paper prototypes. The testing will be therefore focused on the checkout experience, filling forms and so on, which makes me wonder if is it a good idea to test paper prototypes on a so focused aspect of the website.

Do you think is still a good idea? Any tips on how to setup the tasks?