What is their return policy? If they have a satisfaction guarantee, you can always just take her back they can't really refuse to let you return her.
That snake definitely lacks the key features of a yellowbelly. Do you know if they produced her, or if they purchased her from someone else? They might be in the dark on how to identify a yellowbelly. Either way, I would take along a copy of something describing a yellowbelly, and pictures of adult, juvenile and hatchling yellowbellies, so that you are prepared in case they decide to contend you.