Hi,
Has the vet spoken directly with the store?
I don't see how they can justify selling you a sick animal if that turns out to be the case once the culture is back.
If it is sitting in the conditions it got sick in for 5 days with no treatment it is almost certainly going to be symptomatic at the end of it if it is indeed an RI.
I have no idea on the procedure over there but if the vet said it is sick and the store disagreed and wouldn't refund/replace after only 24 hours I'd be contacting the animal welfare people, the trading standards people and the police if it came to it.
But until the vets results are in I don't know what would be the best course.
dr del