i would say bring her to the vet again. If you cannot afford the vet, then i would say try to give her to somebody that knows about snakes and can rehab her. It would be a shame bringing her back to the petshop, you know she is going to end up dying if she goes back there.
I wish you the best of luck, and if worse comes to worse, take her back to the vet to have her euthanized (sp?)