About the melatonin: although available only by prescription in some European countries (I don't know which ones), we've never had anyone on this forum report a problem with European officials over this. And there shouldn't be a problem re-entering the US with melatonin since it's available over the counter here.
Whether melatonin is actually as effective as some users think it is, that's another question, but you're not asking about that so I won't get into the details--except to mention that there was a recent discussion of that subject here. If you're interested in finding that discussion,
go here and follow the instructions for searching this forum
or let me know and I'll find it for you.