Is this against the law?

about once a week at 4am, me and some friends have been driving around and stealing those big blue US Postal mailboxes they have on corners. afterwards, we steal any packages or birthday cars with money, then dump the box in the river(except for one, which is in my bedroom as an ornament). this started last year about a month before Christmas, we made about $3,500 in taking money from Christmas cards and have been doing it ever since. is it illegal?