It actually looks like it’s a pentagram commonly used as a symbol for pagans and neopagans (Wiccans), which are nature religions and, unless perverted, are relatively harmless. Wiccans don’t acknowledge Satan.
You might be thinking of the inverted or upside-down version of this symbol, which signifies Satanism to most of its followers.
I wouldn’t worry about it. It’s probably just a customer’s, or a girl experimenting with her beliefs.