We don't support payments yet. Using a redirect would be your best bet.
We have this listed in the idea section, you should vote for it if you haven't already: http://forums.adobe.com/ideas/1025
FormsCentral now provides the ability to integrate your form with PayPal. In one seamless experience, respondents can fill out the form and pay using a credit card or PayPal account.
You can read this tutorial on enabling Payment Processing to learn more.
We also have created a dedicated community forum Payments to help better support you.
Please tell us what you think.