Amazon doesn't have a "Buy Now" Paypal button. They aren't set up to automatically bill your Paypal account. BUT if you have a Paypal debit card, it's easy to use that as a accepted method of paying for your Amazon purchases. You use it just like any other debit or credit card, and the money comes directly out of your Paypal account. PLUS, by using the Paypal debit card this way, you will get a small bonus credit back from Paypal! I do this all the time so I know it will work.
Another thing to use it the virtual debit card from Paypal, which involves downloading a program and is used similar to a debit card. I downloaded this, but I had so many problems with my computer after doing so that I uninstalled it, and everything has been fine w/ my computer since.