There appear to have been two issues:
- We were processing transactions, but they were not appearing in logs due to a database issue; as @edp noted above, this is now resolved;
- Our system was generating thank-you/notification emails, but Amazon was refusing to actually send them, as we'd reached our sending quota for the day due to the above issue that was spewing out error emails to our sysadmin; we're working to clear this up with Amazon, at which point any backlogged emails should start churning out automatically.
So the good news is, none of our clients will need to do anything about the affected transactions; they should all show up in logs now, and emails should all go out in due course.
We apologize to everyone for the inconvenience and concern, with thanks for your patience and understanding.