Adding message to completed payment screen?
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Unfortunately there's no way to modify the PayPal Screens to do what you have in mind, that would be way too easy.
And you don't just want to display a password on a return URL. There's no security using that method. What you need to do use PayPal's IPN feature whereas you're notified of the transaction, then your server sends an automated email to the buyer with info they need. Similar function as folks do with digital downloads. You can learn more about using IPN here:
https://www.paypalobjects.com/webstatic/en_US/developer/docs/pdf/ipnguide.pdf
Note, if you're not into programming your own stuff then you would need to use a 3rd party service setup to do what you have in mind. This service may work: http://www.e-junkie.com/ Your other option would be to hire a developer, that way you get exactly what you have in mind.
You can find other carts here:

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