I found this fantastic article on John Mitchells blog

HowTo: put PayPal Buy Now buttons in your DotNetNuke Portal

You can do this with a simple javascript trick.

Step 1

Remove the <form></form> tags from your PayPal “Buy Now” code.

Step 2

Add onClick="this.form.action='https://www.paypal.com/cgi-bin/webscr' this.form.submit();" to the buy now input button.

Step 3

Paste what is left into the html module.

What it does is change the action for the Form that is already embedded into the DotNetNuke default.aspx page.