Please don't buy a pig from a pet store. Not only does the pet store keep them in horrible conditions, but the places they're bred are beyond awful. Sows are kept continually pregnant, they get no medical care, they're shipped stuffed in huge plastic containers with no hay and no food. PETA has sued both Petsmart and Petco for the godawful way they treat their animals. If you want to know how they're really kept, see
PetSmart Cruelty.
Pet store animals often are missexed and have diseases. There are a bunch of recent threads here about male guinea pigs being pregnant -- the pet stores couldn't tell the difference in the sexes. And there are a bunch more about people who got their pets home with serious illnesses that required vet treatment right off the bat.
If you'll post the name of the city you're closest to, we'll be happy to help you find a guinea pig at a rescue or a shelter. That way you won't be supporting those breeding mills. A good rescue will do its best to make sure you have a healthy, properly-sexed animal.
The pet store cage is too small. However large it is, it's too small. See the Home page of this website for lots of ideas about cage building, way cheaper than $87.
Welcome to the forum. We'd love to see pictures of your pig when you get one.