You need to put an @ in front of their username in order to send them mail. And yes, you can attach several items to an in-game mail.
For the mail to be received the other person must change zones or log in and out. Going through any loading screen is considered changing zones, so going into a building through a door would be enough to trigger a mail update check on your character.
When I sent the mail to my friend the first time, I didn't use a @ in front of their gamer tag and it worked but I'll try this when I get home. Thanks for the help