Yes, all you need to do is:
- Include a Frame control in your WPF application.
- Include a .html file in your WPF application that contains a element with the appropriate data and some JavaScript to automatically post the form when the page is loaded.
- When it is time for payment, have your display the Frame control and set its Url to the relative path of the .html file.
When the user is done paying you can give them a button to close the Frame yourself, or you can subscribe to the Navigated event on the Frame and when the payment processor causes them to navigate back to your fake HTML you can automatically close the Frame and use your own WPF UI to tell them "congratulations, thanks for paying".