For anyone looking to automate reward/incentive delivery after a Qualtrics survey — the End of Survey → "Redirect to a URL" option already carries what you need:
?email=${e://Field/email} (embedded data) or ?email=${q://QID#/ChoiceTextEntryValue} (a question response) gets the participant's email to whatever page you redirect to.
The part I couldn't find off the shelf was the receiving side — checking the request is genuine, picking the next reward code, emailing it, and never double-issuing on a reload. I ended up building that as a small standalone tool (REWARDR) for my own study: https://ersan.eu/qr-sistem/qualtrics.html — sharing in case it's useful to someone else here too.
