We charge $15-25 extra per room or so depending on how bad it is, doesnt add much time at all. Spray it pretty heavy, agitate a bit with brush if needed... most of the time its not even needed, then rinse. Done this in homes where carpet and filtration is easily 15-20yrs aged. Of course it will eventually come back but from repeat customers usually it still hasnt returned after several years so I would say thats well worth the money spent.
If they decline just do 1 room for free then show them.
If they decline just do 1 room for free then show them.