Rub your nose really hard with abrasive material before entering the store, like those brown paper towels. Make it nice and red. Droop your eyes a little bit, mess up your hair. Try to look really sick. Walk into the store, and pick up a box of Kleenex, throat lozenges, your cough syrup, tylenol, and maybe a magazine if they have one that interests you. Look really sick, play it up. Have fun with it. Don't smile or anything, just place your items on the counter and stand there. She'll ring up your items without a hitch and you can go on your merry way. Just don't break out the happy dance until you leave the store.
Lol fuck that, it's not like they can make you give it back just because you danced after purchasing it.