This is an incredibly sweet idea. "Open when you need to know how much I love you; open when you don't feel beautiful; open when you need a laugh; open when you miss me; open when you're mad at me; open when you need a date night, etc.
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.