That seem like an awful lot of pink stuff! And a pretty high price for a Winterizing, but then again, you're up in MI, and I'm down in NC.
Are you draining the water heater, and by-passing it?
edit: Oops, I thought I read 21-1/2 gallons:S.......2.5 gallons is about right for the flush, plus some in the drains. For what it's worth, I think they get $135 the winterizing package locally....still worth it to do it yourself.
How about blowing out the water lines with some compressed air (about 25-40 psi) and THEN pump the pink stuff through the system. I'm guessing, but you should be using less than 5-gals. I only need 2 gallons to winterize mine, and while it's shorter, it's still a pretty good run from the tank to the galley sink, plus there is the shower, bathroom sink & toilet, as well as an outside shower.
I usually buy 3 gallons at Walmart, for about $6 or so.....and use the 3rd gallon for the drains and the tanks(so the drain valves don't freeze up)
By flushing out the lines with compressed air, you are getting rid of the water, which would dilute the anti-freeze.....so this way, when you pump the pink stuff in, it's pretty much full strength!:)
Good luck