I would keep it in it's carton too. It just makes the entire explanation thing if asked so much easier (because nobody would ever think to empty the carton and put anything else in there ) if you are asked. White crystal in a baggie - must investigate. White crystal in a jar or carton that says epson salt - no problem.

I really wouldn't worry too much about it, even if they take it or hassle you about it, which I don't think they will care, the stuff is available pretty much anywhere without any problems.