Well, I did get an email from Marriott (which has the tracking number) stating clearly that the second night would be free and I would be charged the lower rate on the first night. I'm happy with that. If worse comes to worse and the hotel gives me guff, I will get Marriott corporate to adjust it afterwards.

I don't know other people's experience, but as top tier in five different hotel chains Marriott's email customer service is the quickest and the best. I'm glad they invested in that.