I've seen the hunched position on guppys. And it always ends with the fish dying a few days or weeks after.
Never new if it was a specific disease or just a common guppy sign of poor health.
But take the advice and take her back to the shop.
if you really want to keep her bear in mind that she will eventually die. Set her up in a quarantine tank so she does not spread whatever she has to others.
If its not about a refund keep her. Wether she dies or not she will be much better taken care by you than in the shop and might last a bit longer after all a guppy is not that expensive, but do not keep her with the other fish.
Has a fishkeeper of course I do not like to waste my money ob greedy LFS but even a sick fish deserves to live or die with the dignity of proper care.