One method is use a text or hyperlink.
You can build your own Button Text Links from scratch or take advantage of the Online Button Creator within your PayPal Account. After generating the button code online, select the "Email" tab to view the Text Link Code. Note, the "Email" tab will not be available if have included dropdown selections or text boxes in your item button code. This is because you cannot include dropdown selections or text boxes in a Text or Hyperlink.
Then insert the the text or hyperlink into your page.
you can do that on facebook? i have a facebook fan page i would like to add a button. only it doesn't look like facebook would allow you to do that. where would you put it?