Try some smooth talk

"You're much more beautiful than the normal clerk," you say smiling.

The girl looks up with an annoyed look on her face. But after confirming that you're not some pathetic slob, she raises an eyebrow and licks her lips.

"It's my uncle's store," she replies now standing up taller and pushing her small breasts outward. "I'm visiting him and my auntie for the week, so he asked me to watch the store for a few hours." Her eyes begin to roam over your body.

You lean forward towards her, against the counter. "Well, that doesn't sound like very much fun," you say emphasizing the last word. "Are your auntie or uncle around?"

"Nah," she replies with a wink, "just little ol' lonesome me, bored out a ma' damn mind" she flashes you a sexy smile. "Know anything fun to do?"