Kate was the catalyst all along

Consider this: Kate, subconsciously summoned Jack, and Cash was an angel (albeit with a gun) making this all come together.

Who tracks down an old boyfriend (or girlfriend) from 13 years past just to give him a box with some old cologne, a book, and some pictures? It wasn't like there was his birth certificate or his grandmothers jewels in that box. Most people would have gotten rid of those items by now, or would have kept them ask keepsakes. Most would not think to give them back after all these years UNLESS, they were deeply curious about the person, and thought of the possibility of getting back with them.

As convincing as Kate was, as she was about to board the plane at the end, I believe she intended on possibly re-kindling her relationship with Jack. Perhaps all she needed was Jack to express his desire for her to stay, for her to make the solid decision to get back with him.

That is my interpretation anyway.

I LOVE this movie.
One of my favorites.
The theories here are all insightful.