Here's how I cracked Paypal ...
Once I had discovered that the client's Paypal Account was a "Website Payments Standard" one then I could figure out how to send requests to Paypal.
(I found this out by logging into the Client's Paypal account and clicking on "My Business Setup")
You setup Buttons within your account, that you copy and paste the HTML code for and place them on your XPage source.
(Click on Profile, My Selling Preferences, Paypal Buttons - update)
Adding the HTML code to the XPage source didn't do anything when you clicked on the Buy Now icon ... until I found that there's an <xp:form> tag that is used in conjunction with an HTML <Form> tag ...
There doesn't seem to be any relation between the XPage and HTML Form tags, just that the XPage needs an xp:form one in order to recognise the HTML one.
Really making progress now! I moved the HTML Form Tag to a "confirmation" XPage that was called from the 1st XPage's Submit button, so that the data source can be saved in the first XPage, and then the Submit of the 2nd page can be sent to Paypal.
Hope that helps someone in the future ... and me remember it in a few month's time too! Cheers. Jerry.