Honestly, at this point, I would probably keep the dog. I mean, there is no guarantee that ANY animal your DD gets is going to be "maintenance free." I bought a Jack Russell Terrier and a few weeks later she start sneezing, having runny eyes, looking sick. It turned out that she somehow, someway got a sinus infection. At first my vet thought she might have a blockage in the nasal passage since it was so small. So I had to pay for an x-ray. After that, it was antibiotics. I was out $300.
It's very likely that this puppy is just picked up something because it has been living at a pet store (not the ideal place for a puppy).
What I would do is take the vet bills back to the pet store, explain that the dog was purchased and supposed to be healthy but came home sick. At the very least, they should repay her vet bills.
Again, sending the dog back and getting another is not a guarantee. Now, I don't mean this in a snarky way, but animals can be very expensive, and your DD probably should not have one yet if she has not financial stability (I'm sure you know this). Any animal can have it's share of things. I have owned 5 dogs so far and I can't think of one that hasn't set me back even very early on. Usually some fluke of a thing.