I would like the customer to be male. The salesgirl and him are in their early twenties.
He walks into a large clothing/department store near closing time. He asks the girl how long before the store closes. She tells him not to worry, he can stay after closing time if he wants as she will be working late. After all the other staff and customers have left they go to the changing area. There are 2 rows of cubicles opposite each other, each cubicle has a curtain for privacy.
Either the girl is standing just outside the cubicle watching and flirting with him as he is changing
or:she also tries on clothes in a cubicle opposite him and she leaves the curtain slightly open so he can see her changing.
Either way they end up having sex.
I hope these are enough details. Look forward to seeing the story.
