The best way to do this will depend on what you would like your user interaction to be like. Would you like the user to make the payment before or after the message is sent?
I think the best way to do this would be to use PayPal Express Checkout to process the transaction first. Then you can have the form for the user to fill out on the checkout success page.
We do not have a single product that will create the experience for you but between a couple of our tools you should be able to create an experience like this. Having just a buy now button and a form can be tricky because of the fact that you will not know the status of the transaction at the time. If you need to verify the status of the transaction first then you should use Express Checkout or Payments Pro.