Well...I've added DME about 4 days prior to travel, but obviously didn't get my packet in time. But, we could still take the bus...just brought our bags to the bus with us and stopped at the DME counter to get vouchers printed out.
And I forgot to book DME for my trip in early Dec...booked it about 2 weeks ago, and got the packet about 8 days after I booked.
So, yeah, you need to give them 10 days notice if you want the packet. But, you can book the day before travel if need be. Then, just go to baggage claim, get your bags, head to DME counter and get vouchers printed out, and off to the bus you go. Or you can bypass baggage claim, head to DME checkin, give them baggage info and claim receipts and they will take care of the bags for you.