You will not be able to transmit information via the IFrame, especially if it is in a different domain (due to security reasons). Read about the same domain policy .
Using paypal and google checkout usually means that you redirect the user to the verification site after the redirection to your site is complete.
Another popular way to interact with such sites is to use their APIs (usually open as web services). This allows you to stay on your site and communicate with cashiers - this works more, since you need to develop your own verification and create a communication level.
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