can someone help use sandboxx?

basically my boyfriend is going into bmt today and i wanna be able to write him once i know his address. the problem is, sandboxx confuses the hell out of me.

context, i have a friend who graduates soon and i was trying to send her a letter. i got her address, put it in the app, wrote the letter, and it said it was delivered. i thought she got it, but then the whole letter i wrote to her was delivered to me??? is that supposed to happen? am i supposed to drop off the forwarded letter to the mail? please help.