For those who want to learn:
Framed forwarding (also known as 'masked' or 'cloaked' forwarding) uses HTML code which opens your web site into a frame in the visitors browser. The net result of this process is that your web site opens up your destination URL (such as http://myfreespace.com) in the browser window but your domain name (eg. http://www.yourdomain.co.uk) is displayed in the visitors browser address bar.
PayPal does not allow the use of their payment pages to be displayed inside of a Frame. For security reasons, PayPal members are warned never to enter their PayPal account password into any page whose URL does not begin with https://www.paypal.com/.
When the PayPal logos are used in a Frame, the PayPal URL is not displayed, nor is the security lock displayed in the browser. This can also cause Run Time errors for most browsers if using Javascript on the web site. According to the PayPal User Agreement you are prohibited from using a PayPal logo to display a PayPal payment page of any kind inside of a Framed Page.
There is good news and a workaround. Simply add target="paypal" in the <form action> tag of the HTML code. This will prevent the PayPal payment page from being displayed in a Frame. The corrected line of code should look like this:
<form target="paypal" action="https://www.paypal.com/cgi-bin/webscr" method="post">
Suggest editiong your item button code. After you do that, delete your web browser's cookies, histoyr and temp files. Then close your web browser, reopen it and then test the code again.
Another tip for testing. Whenever you generate new item button code, first copy and paste the code to a totally blank web page, not the page you intend to use it on. Save the blank page with the button code and then test the code to make sure it works OK by itself. Now that you know you have a working button, copy it to your actual web page. If the code fails there, there, the problem is with your web site or page, not PayPal.
(Don't forget the kudos.)