I'm not totally certain, but I think you have to have your dues paid by the time you check in or they will make you pay them at check in. Since you check in on the 15th which is the due date, I think you can pay them at that time and be ok. I do not believe they will let you check in without them being paid, though.
I have a reservation on the 26th and went ahead and paid them because I didn't want any problems with them cancelling my reservation.