I'm praying! Agree that parvo is immunity that differs with the pup, depending on the weaning time and stress of both puppy and mother/others.... Our dog was given a series of parvo shots, not just one as the breeder knew that immunity factors vary and the higher cost was worth it to him and us to do it this way.
I still think you should be in touch with Department of Agriculture and share with them about this being the 5th puppy......I don't mean to "knock" pet stores but they are just often notorious on buying cheap and selling high.... I know that we sell many bunnies to people that both bunnies 3 times more expensive than ours to have them die within 2 days!
It's a shame.....Please for the sake of future buyers and the puppies, report this and then let the "state" authorities decide what to do.....I for sure don't know the guidelines that the pet stores operate under.