Ok, I feel the need to be the practical one here... I think you should return the pig and adopt from a rescue instead. Let me explain:
If the guinea pig you bought is showing signs of poor health and the one you already returned was also in poor health, it is probably an indicator that they will continue to have poor health. And I agree with everyone, the store vet is not going to help. So maybe it is best that you leave those girls at the store. If you keep them/buy them back, you may be setting yourself up for a lifetime of dealing with and paying for sick pigs. I hope that doesn't sound mean because I know how quickly we can get attached to them. But you have to be practical too.
(I'm sure everyone will now jump on me for suggesting you "return" it. But the pig is no worse off, some other unknowing person will buy that pig.)
If you can contact a rescue in your area, they will be very willing to help you find a pair of healthy pigs that is best for YOU. Unlike the pet stores who are just interested in your money. A rescue provides better care and vet care, so you'll get healthy pigs. And a rescue is much more knowledgeable about the personality of each pig, so you can get a friendly sociable pair. (your experience with the employee who told you crusty ears are normal should be a sign of how little the employees know about pigs in general, much less each specific pig).
Best of luck!