Normally you don't "purchase" exam vouchers. Vouchers are given to you as a part of some other deal, for instance, some training manuals have an attached voucher for a discount in the price of the exam.
To buy an exam, you go into www.prometric.com/microsoft and follow the directions to sign up. At the end of the process, you are presented with the price of the exam and the payment options. At this point,
if you already have a voucher, you enter the code on this screen. If you don't have a voucher, then you provide a credit card number to pay for the exam. And that's it, you don't get to buy the voucher at any time.
A different thing that you can do is to buy a "pack" of exams, or sometimes a second shot when the offer is available. Depending on the type of offer and how it is organized, you may be actually getting one or more vouchers for your money. These offers are
linked from the Prometric page.
