As others have stated the Express is a package targeted at owners that don't necessarily care so much about bells and whistles as they do about the overall appearance, and performance. It's really smart marketing by RAM, considering the sheer amount of Express package RAM trucks I see on the roads every given day. A lot of people love the look of the Sport. The front fascia and rear bumper are all color matched, but to get into a Sport you're looking at some major money as a Sport includes a lot of creature comfort type options, and a leather interior. The Express is an option for buyers getting a Sport type look, without having to spend Sport type money. To my knowledge the RAM box is not available on Express trucks. I don't think you can even order one with the RAM boxes, though I could be wrong in that regard.
I personally didn't much care for the Express when I drove a Black Edition. It just felt cheap to me. No offense to Express owners, it just wasn't my cup of tea. I wanted the Uconnect 8.4 and you can't even order an Express with that, or so I was told. I also wanted some of the options that simply aren't available on an Express so that's why I ultimately ended up deciding on my Big Horn.