It is illegal to mail alcohol through USPS. However, you can mail it through UPS or FedEx. I wouldn't broadcast it though because, while it's not illegal through their services, they're not too keen on it.
I've done a couple trades through Beer Advocate, and they've all gone well. I've typically thrown in a couple extras into the box. Don't go overboard with that though if you choose to.
Make sure you pack the box with caution though.
Drinking: APA, My Own Personal Helles
On Deck: American Wheat