Personally, I would go and get it, because I'm a soft touch, and couldn't leave "my" kitten somewhere like that. Certainly take it straight to the vet, and certainly report her, but please go and rescue your kitten.
We have had several cats. One was brought to a pet shop from such a place like this, in a shoe box, with several others, and sold to the pet shop because they hadn't sold online. Not the best start in life. We took him straight to the vet, who had heard bad things about that pet shop. Anyway, 5 years later he is still a happy, healthy, loyal, loving pet. He has a heart defect, which may be down to inbreeding. We have discussed it with the vet, and have decided there is no need to treat it at all, although it might or might not in the long term shorten his life.
Anyway, he is the only one of our cats who wasn't a "rescue" cat, but the vet said we did rescue him, but not officially.