Do not ship via USPS, period. It's a felony to do so. Odds of getting caught are fairly small, and odds of being charged smaller, but it's potentially a year in prison if you get caught (and in the even more unlikely event that your shipment injures someone, penalty can go up from there, as far as life in prison or even death penalty if it kills someone). It's not worth the risk.
If you ship via UPS or FedEx, like others have said don't tell em what you're sending. It's against their policies, but not against the law. Worst case scenario, they learn what you're shipping, halt shipment, and don't refund your money. Most likely the person who found beer drank the beer.
Even then, I shipped via UPS for years, and have had two issues. One broken bottle, where they removed the damaged bottle, repackaged the whole shipment and sent it on its way to its destination (even though against policy), and another (my most recent shipment for the record) where nothing was broken, but random inspection (which they do to keep their hands clean in the event they ship contraband) caused them to halt my shipment (of some damned nice beers for the record) and didn't refund my money. I wasn't happy about that. I'll be switching to FedEx after that.
Unfortunately there's no truly up and up way to do it, unless you want to pay a private courier to do it for you who knows what they're shipping, and you'll have to pay big buck for it.
So go UPS or FedEx, don't tell em what you're sending (print the label out at home, attach, and just drop off, they'll seldom ask questions), and perhaps doing something to disguise the package.