Depends, is the user normally redirected directly to the Paypal site after clicking the "Buy Now" button?
In that case, you could have a "Buy Now" button that points to redirect.php
You can then have redirect.php send an e-mail to you@yourdomain.ext.
You include in the header:
<META http-equiv="REFRESH" content="0; url=http://www.paypal.com?paypalarguments...">
I'm not entirely sure that this will work, you'll have to experiment with the refresh meta tag to make sure that the page has finished loading (and the php script has finished executing) before it loads the paypal page.
