When paying through Paypal, Paypal would rather not have you pay using a debit or credit card. Paypal prefers that you either use your Paypal account or open one if you don't have one. So they make it a little tricky. Here are the tricks…
First, do not Log In to Paypal. Instead, click "Pay with a debit or credit card".
After you fill in your card information, they will compare your information with that in their Paypal account database. If you have a Paypal account, they will probably find it, and will try again to change your mind with this:
Again, do not Log In to Paypal. Instead, click "Continue without logging in".
If all this fails, you may either go back and select Google Checkout instead, or else enter an order via email and pay by mail.