Replying but alas can't help much with paypal sandbox.
I'm working on a different extension to allow a per item (course, quiz, quiz attempt, etc.) paypal payment. Obviously this extension does not aim at using the enrollment method which is limited to courses but I can look into adding that. As for per item, last time I checked with paypal they did not have api for a granular IPN which would be required for a per item payment confirmation and activation but they said that they were considering it due to increasing demand. So if the extension is completed in the near future it is likely to require manual activation of purchased items.