The first step of checkout is a pre-qualification (soft check) performed by Citizens Pay. This will not impact your credit score.
If you decide you want to purchase a device, then there will be one-time hard credit check. This is the very last step of checkout and you'll see a note about it the checkout flow.
A hard check will only happen once within a 30 day period. So you don't have to worry about going through the checkout flow a few times.
And, if you already have an open line of credit, then there are no more credit checks, if you make a second purchase it will just use your existing credit.