I have some experience of using paypal with an asp.net website, however this issue has me really stumped.
Root of the problem: You cant embed the html form for the payp
Submit the PayPal information using their APIs rather than submitting a form directly to them.
That way you can keep everything as part of a single form and add a little more robustness around the PayPal input and validation.
PayPal: SDKs and Downloads