I found this fantastic article on John Mitchells blog
HowTo: put PayPal Buy Now buttons in your DotNetNuke Portal
<form></form> tags from your PayPal “Buy Now” code.
onClick="this.form.action='https://www.paypal.com/cgi-bin/webscr' this.form.submit();" to the buy now input button.
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.