You can actually make DME reservations the day before you travel. BUT...you obviously won't get your packet with vouchers and luggage tags. You need to book at least 10 days prior to travel in order to get that packet. If you don't get the packet, you will have to stop at the DME checkin counter and get vouchers printed up for your bus ride. And you will have to either go to baggage claim yourself and get your own bags and bring them to the DME bus with you, or you can wait until you get to the checkin counter and give the DME people your baggage claim receipts and a description of your bags and they will send someone to get your bags and deliver them to your resort.
I would seriously consider booking DME now. That way, you'll get your packet. You can give them the actual info later on, when you've booked flights.,