Instead of mailing it and taking the chance that it will get lost in the mail even though you are not far away, what about wrapping it up and leaving it in his car when it is finished. Could take him a day or maybe even a week to discover that it is there!!!
Another idea----make him work for it! Make him go on a scavenger hunt....could be a two or three day ordeal! Just let him travel or search all over with lots of MYSTERIOUS clues until he finds it. I would love it more that longer that it takes him to find it..