Take advantage of Paypal's auto-fill.
I just had to enter all my shipping info, and then found out that Paypal is a payment option. Do you realize that PayPal will auto-fill all the information you ask for?
Do you realize that internet merchants take advantage of this to make their site more user friendly and impulse buys (like my last one) easier?
All you need to do is offer the option of "Check out with Paypal", and let the information come to you from Paypal.