Its illegal to mail beer through the US Postal Service (USPS).
Fedex and UPS are not government agencies, so although it is agains their policy, its not illegal to ship beer through them. Just don't tell them you are mailing beer.
I personally like Fedex better. IMO, UPS gets too nosy and have even gone as far to open my boxes to see what is inside. I just take a sealed box to Fedex, they weigh it, I pay and am out the door. I sometimes tell them I'm mailing picture frames to make sure they "handle with care". Just make sure you wrap them really well in bubble wrap, and give lots of padding around the bottom, sides and top. I've mailed a LOT of beer this way for a multitude of swaps and Secret Santa exchanges. I'd also hesitate to mail more than 6 12 oz bottles in one package. Beer in bottles are pretty heavy, which gets expensive really fast!