Just did the process, I went to the apple reservation site:
https://reserve.cdn-apple.com/US/en_US/reserve/iPhone/availability?iPP=U
then clicked the store, the model, the size and color, and hit continue, it then told me to sign in with my apple ID.
then it told me to text a code to a number and i'll get a code back to confirm it along with my number, it then gave me which times were available for me to pick up said phone, with only 1 button left "Confirm Reservation" I figured it would take me to the screen you stated but it just confirmed the reservation right then and there. I believe it is because I chose the t-mobile version, I saw on the apple site that you need to do it in-store if your carrier is t-mobile.
was really hoping to do it online so it makes life easier for everyone, thanks for the help anyways!
EDIT: I believe you were using the checkout page instead of the reservation page because I found what you were talking about while i was simulating a checkout, as opposed to reserving one on the link I posted.