Well, I would never just let the yard do what they want to do and write them a check later. At the very least point out to them the systems that need to be winterized and discuss some specifics, like bypassing and draining the water heater rather than just dumping an additional 6 gallons of antifreeze into it which BTW is a bear to get all out in the spring.
But this is really a pretty easy DIY job. A summary:
Engine and generator- Close the thruhull, open the top of the strainer, start the engine and dump 2-5 gallons (2 for sailboat size engines, 5 for big 6 cyl ones) of antifreeze in while watching what comes out for a strong pink color.
Air conditioning- Remove suction hose from strainer, elevate it and with the A/C on dump a gallon in the hose with a funnel.
Fresh water system- Drain tank. Disconnect pump suction hose and put it (splice a hose if necessary for length) into a bucket filled with antifreeze. With the pump on, open each fixture until string pink comes out. For the hot water system, bypass the hot water heater with a coupler fitting or a short length of hose and some clamps. Drain the hot water tank to bilge. Open the fixtures as above.
Sanitary systems- Dump holding tank. Flush with water and flush macerator pump. Remove suction hose from thruhull and put it in a bucket of antifreeze. Run or pump toilet until bowl is pink. Pull discharge hose off of macerator pump and pour a cup into pump. Pour a quart or so down the shower and a cup into each sink trap.
Bilge- Pump out until little water left and dump a gallon of antifreeze in bilge to protect bilge pump.
After the boat is hauled, open each thruhull and let it drain, close. This eliminates the pocket of water in the ball that can freeze.
I suspect I have missed a thing or two, but that's the general procedure. I can do my boat in about an hour. It takes about 10 gallons of antifreeze- the -50F pink stuff of course.
David