If you shipped the item within the time you specify in the listing, then DO NOT WORRY, I really feel that some customers do this with the nasty intention of getting a refund or at least the shipping cost back.
DO NOT refund any money if you shipped in time and you have proof of shipment.
I´ve receive some ugly uuuugly emails with customers saying things like I ruined their perfect reputation because it was a gift, bla bla bla.... really, not joking. But my policies and shipping procedures are clearly stated all over my shop.
You did the right thing, just wait, he´ll probably send you several other emails, just stay cool, find out what you can at the postal office but like I said, if you shipped in time and your listings and policies are clear, just do not worry, he is absolutely trying to get some money back or needs his medication (LOL)!!!!
Best wishes =o)
Laura