you have to do them trad, or with a cleaning solution and a lint free cloth.
I prefer trad way.
Learn the methods and use all the tools of the trade.
I charge the same as i do them outside, if its at the same time. If the sills have not been emptied of all there photo frames etc, then i charge an extra 20-30%. Bear in mind that i advise any breakages caused by me, are not covered! (its a good way of getting them to remove items from the sills).
Some may charge 1.5 or even 2 times as much to do the insides.