The customer, Alisa Reed, appeared in court for the first time on Thursday in Cumberland County. She is facing multiple charges including attempted first-degree murder.
Police Chief Errol Jarman called it "a senseless act of violence" that now a child as well as her family and friends will have to heal from.
The shooting happened at the Subway on Murchison Road on Wednesday evening. Police said the 37-year-old Reed got into a verbal altercation with a number of Subway workers, saying they got her order wrong. Reed got upset and began walking out of the restaurant.
Authorities said a 15-year-old female employee was told to lock the door behind Reed who then pulled out a gun and started shooting, hitting the girl. There were two other customers inside.
Police said the girl is currently at UNC Chapel Hill and is expected to recover.
Reed is facing charges for attempted first-degree murder, discharging a weapon into an occupied property, assault, and communicating threats. She's also facing a charge for assault with a deadly weapon with intent to kill and inflict serious injury.
She is in custody at the Cumberland County Detention Center with a secured bond of $300,000.
Spring Lake police said they are continuing to investigate and are looking at surveillance footage from restaurants in the area.
Subway has not responded to ABC Raleigh-Durham affiliate WTVD request for comment.