The iframe could work. However...

You do not want the payment-provider in an i-frame. (for one, people need to be able to see the url to verify they aren't being scammed)
Can I create a standalone page using dnn? Or will anything created with dnn always be enclosed in a form?

I guess this will require some webpage not being a part of the dnn site, as a proxy.
If only there were a way to not only create an iframe, but also it's content in one go...
I can write the source url of the iframe containing all paramaters. The only thing that external page has to do is rewite the GET request into a POST request and post it to the payment provider, while escaping the iframe.

[iframe src="proxy.aspx?amount=42&returnto=mydomain.com/succes&signature=234987527389752&postAllThisTo=pay.mastercard.com"]

That page (proxy.aspx) would then output something like [form method=post action=pay.mastercard.com target=_top][input type=hidden name=amount value=42]....[/form][script]form.submit();[/script]

That generic page might come in handy for other projects as well :) It might even already exist somewhere on the internet.

jquery.post (or actually javascripts XMLHttpRequest) can indeed post the data to the payment provider, but it returns the servers response to javascript instead of redirecting the clients browser to the payment provider.