I recently sent a care package to some missionary friends of ours and found an affordable way to send a few treats from home.
Since you can no longer send things SURFACE mail overseas, everything has to go AIR. For about $11 you can send a "Priority Flat Rate Mailing Envelope" to any country overseas. You can fill it pretty full, especially if you take the boxes and packaging off items and close up your bulging envelope with packaging tape. It can weigh up to 4 lbs. Some of the things I recently sent our friends were: books, decaf tea bags, sugar free gum and sugar free drink mixes. I even slid in the comic section of the Sunday paper, decorative napkins and some holiday candy.
You can also send a bit bigger box (about the size of a shoe box) with "Priority Mail" for $41. I realize it isn't cheap, but you can get a lot in that little box, espeically if you take things out of their packages and boxes. Again, I would get the box ahead of time so you can fill it to it's max at home, then tape it up and bring it to the post office to send.
It's a good idea to e-mail the missionary before hand to see what they would like and that their mail gets to them safely. Make sure that they don't have to pay an expensive customs tax to receive the package. Also, be careful sending items that could melt in summer's heat. You can put candy or lotions in zip lock bags to protect the other items in the envelope. At the post office you'll have to fill out the customs form (pictured). You can request the mailing envelope from the post office ahead of time so that you can fill it up with goodies at home, then bring it back to the post office to pay for it and mail it.
Often missionaries would like to order books on the internet that you can send them...or maybe they would like some medicine or personal items from Walmart..or spices or a favorite candy or magazine... or a new praise CD or family movie on DVD.
Another idea is to send gift cards when a missionary is coming home on Home Assignment. We had a supporting church buy gfit cards from different stores so we could buy needed clothes, household items and school supplies when we first arrived back in the U.S. with big shopping lists and not-so-big wallets.
So, think of sending your missionary friend a small package to reminding them that you care!