Source: Mitchel Sellers Blog

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’;his.form.submit();return false;”

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.

Note that in Step 2 I added a ‘return false;’. This is because intermittently the page is submitted twice… This gets rid of that issue.