There are a variety of ways to buy from the Windows Phone store. You can pay by PayPal, as you mentioned. You can also pay by credit card, or at least on at&t in the US, your purchases can be added to your monthly phone bill. Regardless of your purchase method, the mechanism is more or less the same. You enter your financial information into your Microsoft account ahead of time, and when you actually make store purchases, it's basically one tap to authorize charging your account, and your app downloads.