I'd have petco reimburse any medical costs since you just bought her from there and she came ill.
And ditto @
peachyteen. Look into adopting next time. I'm not surprised the pig you purchased from *shudder* petco is sick. No pet store is good, but I especially dislike petco. Such a dirty store in my opinion.
I was actually going to say that you'll likely have to take the pig back to the store.
When you sign the contract purchasing the pig from the store one of the things you're agreeing to is that if the pig gets sick the pet store gets the first chance to care for the issue. They may offer you a 'replacement pig' but TBH if one has an URI they probably all do.
What I would have done in your situation was taken her to a good cavy vet, told them I was waiving my contract and accepting responsibility, and paid for everything out of pocket. If you do that, though, the pet store will not offer another pet in exchange, and definitely won't reimburse the vet fee as taking her to a vet yourself violates the contract.
Vets used by pet stores are not exotic vets, and have no training in the care of guinea pigs. You need to find an exotic vet who can care for your pet, and then try to get the pet store to pay for it.
This really depends on the store, and the manager. I worked at a PetSmart for 3 years and our animal care manager would take the sick small animals to a vet who specialized in the little guys - hamsters, birds, guinea pigs, etc. She had developed connections with a few vets so she'd pick certain vets for certain species or issues whenever possible.
Sure, not all store are like that, but people don't work in pet stores because they hate animals. they work there because they're broke, but love animals. Usually. (Having worked in the business for 10 years I'm pretty confident in that.)