Amainhobbies seems a little shady!!

I have only purchased parts, not full kits from amain, but they are great.
I live on the east coast but I still get my stuff quick. They ship out same day (at least they have for me) and shipping is quite reasonable.
My only complaint is that the free shipping for over $100 is fedex, so if I spend more then 100 in parts (still a small box) it will take 5-7 days to get here, were as if I pay the $4-5 for first class USPS, or $6-7 for priority mail, it will only take 2-3 days.
Why do companies prefer to deal with UPS/Fedex even though they cost more?
And, if they end up giving you store credit, is that really a bad thing? you know you will end up buying something else...