Zihua Airport and Wheelchairs

Since you mentioned that you need the wc until you get a taxi, I assume you need this when you land? This should be arranged when you check in to get on the plane. The airline will arrange for this service and you will be met at the plane. You will asked if you need it right from your seat in the plane, or if you can deplane yourself (which means quite a few stairs to get down from the plane), in which case the wc would be waiting for you at the bottom of the stairs. If you are departing from Zihua, the same thing applies- make the arrangement with the airline ahead of time and the wc will be there for you when you check in. Someone can also go to the check in counter and get the chair to take it outside and bring you in if you are unable to walk to the check in counter.
This is the same for all airlines and for travelling anywhere. I have never encountered any charge for this service.