Yes, our prayers go out to Enzo! We're all pulling for him. Make sure the vet you are using now has EXPERIENCE with our little Yorkies! I can't stress that enough! It makes all the difference in the world, as we've had members here with Yorkies that got overdoses of medicine because of inexperienced vets!
I would complain to the pet store, but of course not give Enzo back, perhaps try to ask for a partial refund? Or to help pay for some of the vet bills.