Go to Apple.com login with your username and password. Pick the exact phone you want. At the end you will get to a point where it will ask you how you want to pay. Once you select your payment method. It will bring you to another screen. But you wont be able to go further, cause the continue button will be greyed out. But at that point there will be a heart icon to click. Once you click it, when you go back it will be listed in your favorites. So you can go straight to that later. But you have to be logged in, in order to save your phone and payment method.