MotoMail.com It was my best friend. One of my guys was way out (Darkhorse 3/5) and we had virtually no communication with him for 6 months. MotoMail allows us to e-mail them and the e-mails get printed out and delivered. It made me feel good knowing that I could 'talk' to him any time I wanted... though they only got deliveries every few weeks.
Also, yes the USPS lets us ship the flat rate boxes at a discount. I sent tons of stuff, but one call he made in the middle of the night was that he wanted Ramen, Energy Drinks, and Cigarettes and Chewing Tobacco. I was FLOORED as he didn't smoke or chew, but he said he could use those things to trade other guys for things he wanted. lol I refused to buy them, but allowed one of his friends to buy them and I shipped them. Why that seemed justified in my mind I'll never know.
Also, socks and underwear. The only way my guy had to do laundry was by hand, and that was only on occasion. They would wear socks and underwear for days on end. I always alternated a package of each in every package I sent. Oh, and a couple thermals. It get's cold. Magazines were a favorite of his also. Lots of time people get those lists, and so everyone sends the same stuff over and over. Just take your best guess based on what you know about him. Packages take ABOUT three weeks. I sent a package about every other week.
Plan for a lot of what you send, to be left behind. They don't have a lot of space to bring stuff home.
Also, check out MarineParents.com, defendersoffreedom.com just off the top of my head, to get on the list to have packages mailed to him.
Good luck to him and all his loved ones. It's super hard for us back home waiting..., being left back here and always wondering how they're doing.