I read this story on FF.net a while ago I think. It was a future Klaine fic where they were married, but fighting a lot. They wanted to get divorced, but promised each other they would open the box they made when they got married first. I believe they reconciled at the end. Any help would be great.
Edit: 2-19-13- I also remembered that there where letters involved. The each wrote the other a letter, and went to separate rooms to read them. The met back up in the living room, and shared a bottle of wine. thanks again.