I would do a DNA test on the puppy. Contacting the 'breeder' would most likely be a waste of time, unfortunately the majority of puppies in pet stores, are usually out of puppy mills
It's very possible the mother was bred by two different dogs.
I would start with a DNA, I would report the petstore to not only the local AC but to your states Head of Agriculture Dept.
I would also tell the pet store that you ARE going to DNA this puppy, and since you paid ALOT of money for her/him, if the DNA comes back he's 'mixed', your taking them to court.
I would not return the puppy, and I would pay "something" for him/her, but I would want the majority of my purchase price refunded.
The puppy is very lucky to have YOU in his/her corner and every puppy deserves a home, but unfortunately I don't think this puppy is a purebred gsd
He/she may not have any health issues right now, but who knows what the future holds.
Good luck with him/her,,i would nail that petstore to the wall.