I love this idea. Give him a slew of envelopes, each one with a specific condition. Examples are: Open when you are feeling sick. Open when you need motivation. Open when work is aggravating you. Open when you need to remember how much I love you.
Husband gift - better than a note! It won't fall apart! This is something he can carry in his wallet forever. Great small gift idea to give him at our rehearsal dinner. Would be cool to have wedding vows put on it!