When you're creating the button, in the first field you'll enter the ammount to charge and right under it is a dropdown to specify the frequency of the charge. Since it's a one-time payment, go down the list to the options under the heading
PayPal (Buy Now) and select one of those. Then fill out of the rest of the form and post the short code in your page.
When someone clicks the button, he'll be taken to PayPal, where they can login to their account if they have it, but also choose to pay with a card without an account. Right under the login form, you'll see this line:
Let me know if that helps you.