Wouldn't it have been easier...

PRECISELY!! When it came out I thought I must have missed something, since the solution was so obvious. To do it without theft or kidnapping would have been so easy: they obviously had his wife's case from the locker, so knew it was a simple mix-up; they knew where to call, so just do a quick, non-violent swap: "Hi, we'll come on up with your case, OK". They didn't think to look for it? Further, given that it was likely the case would shortly be sent back to the airline, they were very lackadaisical about making demands on Harrison. A couple of missed calls. Send a letter by messenger, fergodsakes. I generally liked the pace and acting, but I HATE it when there is such a glaring fault in the very premise of the whole movie. Yet no one else was talking about this when I was walking out of the theater. I sometimes think that moder movie-makers just figure that we're so dumb that, if they make the plot fast and complicated enough, the audience is so dense that we'll either not catch it at all or figure that we must've missed or mistaken something important.

They initially thought she was the courier and had either cheated them or failed the job.