Pet shops are legally not allowed to sell medication. Their stuff is only geared for prevention.
That said, healthy, well kept guinea pigs with a fully working immune system do not need preventative treatment that can contribute to develop resistance to ivermectin in those mange mite eggs that dormant in the skin as the immune system keeps those eggs nicely under control. It is much better (and overall cheaper) to treat with a full course - for mange mites that is 3-4 rounds - of the appropriate product for the specific skin parasite in an acute case; you always need to treat all piggies in contact. It is also easy to mistake a fungal skin infection for mange mites; they can look very similar in the very early stages, hence why we can never replace a hands-on vet visit.
If you have just bought your guinea pigs, you can reclaim any vet cost from the shop by presenting the vet bill together with the sales receipt.
Here is our recommended vets locator:
Guinea Pig Vet Locator