I think everyone missed it. This moving is about a many who sits in his chair and fantasizes about all the things he might, should, could, have done if he only had the...nerve?, ability? He's unsure of what he lacked in his life, but it's represented in the movie by the mysterious box under his bed that he can't open. He keeps putting it off until tomorrow. It's a symbol of his procrastination, or his inability to do what he must to get on with life. FYI...movie watchers. He never dated that girl, he never killed Hitler, he never saved the world from Bigfoot. He imagined he did all those things.
We never find out what's actually in the box because it represents what's wrong with him. His inability to face it, is why he can't progress. When he returns from his Bigfoot adventure, the picture on the wall in his house looks exactly like the mountain he climbs to find Bigfoot. He made the whole thing up in his head because he was staring at the picture.
At the end, there are no letters, no picture, no bigfoot, no medals. All record of these supposed events are gone. As they would have to be.
But his inability to open the box, even in the last scene...means he is still the same man with the same issues he has never overcome. I think the movie is a commentary on the personal dangers of satisfying one's self with fantasy and self delusion rather than facing real life.