This scene from Five Easy Pieces might work. It involves Jack Nicholson trying to order a meal in a diner, but he wants a few substitutions, which ultimately make the dish something completely different.
The Dead Parrot Sketch from Monty Python’s Flying Circus is pretty funny.
In this other funny clip from Monty Python, Michael Palin is attempting to purchase An Argument but gets sent to the wrong department.
There’s lots of funny clips from the British TV series called Are You Being Served which is set in a department store.
Fast forward to 4:31 in the episode of The Nanny called Rash To Judgement in which Fran develops an ugly all over body rash on the evening of her first romantic date with Mr. Sheffield. The scene in the drugstore, where she goes to buy some ointment is hilarious.
There’s a very funny scene in Chevy Chase’s Christmas Vacation in which Clark (Chevy Chase) has an embarrassing encounter with a beautiful salesclerk.
Fast forward to 6:13 to see another funny episode of The Nanny, called Making Whoopie in which Fran goes to a Chinese herbalist to buy some fertility herbs (as she and Maxell are trying to conceive). The best part is the interaction between Val (Fran’s dim witted best friend) and the store clerk.
I can’t seem to find the clip on Youtube, maybe someone can help, but there is a very funny scene from the movie The Summer of ‘41, in which one of the young fellows, Hermie, tries to buy condoms in the drugstore, but he’s very embarassed and the store clerk doesn’t realize that he is trying to buy condoms and he keeps asking Hermie questions. Eventually, Hermie fesses up to wanting condoms, but then the store clerk asks more questions about what size and how many does he want. The kid, not knowing how many come to a box, asks for a whole box of large sized condoms. Although the store clerk has already caught on to what is happening (potential underaged sex) Hermie tries to convince the clerk that he really wants the condoms to use as water balloons. This scene will probably bring back a lot of memories for men recalling their first time purchasing condoms. It’s very sweet, not dirty or controversial at all.
Not sure if this will work, but there’s a funny scene from the Carol Burnett Show in which Harvey Korman and Tim Conway have purchased first class and no frills seats, respectively, on the same plane. The differences are drastic.
You might be able to use this funny Visa commercial from a few years back starring Yao Ming which points out the necessity of making yourself understood, whether you’re the store clerk or the customer.
Ok, that’s all I’ve got.