It only gets cold enough here to freeze once every 20 years or so, and there's no salt on the roads.
I keep a 1/2 gallon bottle of ammonia in the car. When I want to top up the WWWF, I pour a little in and fill the rest with water with a hose (we don't have hard water here). Presto! So easy, cheap. Does a good enough job. AFAIK, the ammonia is no threat to the system in the car, or the tap water.