If you feel like they can have a better life with someone else, then maybe it's okay to consider surrendering them to a shelter/rescue (I wouldn't return them to the pet store as that will just decrease their chances of getting into another good home with people who truly understand guinea pig care). However, in general, guinea pigs are not that as expensive as say a dog or cat... I'm not sure what your expenses are but if you are finding that you are spending over your budget, my advice would be to buy your hay, your pellets (if you feed them), and your disposable bedding (if you use it), in bulk instead of at the pet store. Buying in bulk allows you to get better quality and a bigger quantity for less. Pet stores overprice everything, in my opinion, and the hay and pellets are never fresh. You get so little for so much money. I'd say the only REALLY expensive part about owning guinea pigs, or for any pet for that matter, is the unpredictable vet bills. But most of the time, depending on where you live and how many piggies you own, veggies, hay, pellets, and supplies are not really much.
Correct me if I'm wrong, but you've only had these guinea pigs for a few weeks....right? They'll get used to you, I promise. Don't give up. Most piggies really learn to warm up to their humans and the patience that is required is definitely paid off in the end. Trust me. Just keep trying and don't get discouraged.
If you feel like guinea pigs are just not right for you, and you feel that they can live a better life with someone else, I wouldn't see anything wrong with giving them up... not that I do anyway but I truly believe that you are doing just fine, especially for a new time cavy owner, when most make a lot of mistakes in the beginning and need to readily adjust how they care for their piggies over time. I really do admire cavy owners who research first and really try their best with their piggies. I think you're a wonderful owner and I'm sure that if you decide to keep your girls, they will love that