Ok so my experience here is that females and males are very hard to sex in guinea pigs, we had babies and couldn't tell until they were fully grown.
We also had to desex the one male baby that we had as we wanted to integrate our 2 herds into one and didn't want incest amongst ours. It cost around $190 to have the guinea pig desexed and it is a dangerous operation for them as they are very sensitive to a lot of drugs and from what I gather one of a guinea pigs hobbies includes dying.
I would definitely be asking the question of the pet shop, and maybe selling the babies to them to fund the desexing if you have become attached to your furry little pets?
Great idea, or give them so the pet shop can fund the desexing and recover costs