Itching and scratching can be caused most likely by either mange mites or fungal infection, usually ringworm. Both are not at all uncommon in highly stressed guinea pigs with not yet fully developed immune systems and exposure at the shop.
Either will require a proper diagnosis and appropriate good quality treatment. Cheap shop products are too low dosed and will only help to make things worse in the long term, as can treating on spec.
If the itching has happened within 2 weeks of getting your piggies from a pet shop, you should be able to reclaim any vet cost by presenting the vet bill and diagnosis together with the sales receipt. Do not let yourself be fobbed off by the shop and by using medical broad spectrum junk products. I have added US based links which list brands that should be available where you are.
Guinea Lynx :: Parasites
Guinea Lynx :: Fungus
Gorgeous Guineas Photo gallery
Gorgeous Guineas Identification
Ringworm: Hygiene And Pictures
As you are mentioning Pet Smart, I assume that you are in the US or Canada? As we have members from all over the world, please add your country, state or province to your details, so we can help you much more effectively. Click on your username on the top bar, then go to personal details and scroll down to location.
Here is a link with recommended vets for the US and Canada
Veterinarians - The GLX-Files