While the hotel is clearly dropping the ball here, in past experience if I've had a billing problem noticed after checkout, a call or email to the Marriott Business Services at 866-435-7627 usually takes care of it. They do the legwork and coordination with the hotel for you and are really, really fast.
I had gotten behind on my expense reports and when I went to do them some 6 months hence, I found I was billed for a night after I had checked out and also charged for the internet all week when I used it once. The charges were refunded within hours and without me faxing in proof, they have everything at their fingertips.
Just the best customer service I've seen in a long time...