Age issues aside, imho if your gut reaction to a site is skepticism then you are probably right to avoid buying from them (where ever they are located). Would you hand your CC info over to some shifty dude in a run down store in an unfamiliar part of town? Probably not. Treat this the same way. You are taking a risk in any online purchases and although there are safeguards by using a credit card, wouldn't you just rather avoid the hassle?
I can't give you any info about Performance Airsoft because I have never used them or heard of them till this point (low post count alert!). If I was in your shoes, I would just get your cousin to order himself something in your price range and flip him some cash. You still come off like a star and at no risk to you. taa daa.