The day before you leave you will get an envelope on your door with a letter telling you what time your DME bus pickup time will be. At the bottom of that letter is a number to call. Have a pen and piece of paper ready, because as soon as they answer the lady will immediately ask how many bags you have to check, ask for your credit card number, and will very quickly give you a confirmation number. The lady I got this past May when I called, talked so fast I barely had time to write the confirmation number down, before she hung up.
Now if you happen to be overweight the next morning when you check your bags in, I'm not sure where you pay the overweight fees, but I think you may have to take them yourself to the airport and pay there. When I was checking in at RAC at POFQ this past May, there was a couple off to the side, rearranging stuff in their bags to get under the weight limit and I heard the RAC agent saying something about they could not take any money there and would have to pay at the airport if they couldn't get the bags under the weight limit.