i would just act like you are buying it for a friend at school who is to scared to buy it her self. its an old trick but people do it all the time. when i was a manger at Food City i would over hear the cashiers talking about how they would get their friends to buy them one or how someone just came threw their line that was buying a pregnancy test for someone else. just do it that way. but play it off well or they will know its for you.
