Hi and welcome
Please have her vet checked, so you can treat exactly for what she suffers from. From your description we can only guess whether it is mange mites, ringworm (fungal infection) or hay mites (whose tiny egg cases are visible). Fungal and mites need different treatment but in any case better one than cheap low dosed broad spectrum shop stuff that is not effective in clearing an acute case.
You can reclaim any vet cost from the shop that has been incurred in the first two weeks after the sale if you present the vet bill together with the sales receipt as exposure and infection must have happened at the shop. It comes under the heading of selling damaged ware.
Have your two girls been in contact with each other? In this case, you will need to treat both.