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.