DG firms like safestyle just pay commission only to canvassing/sales teams on results after the windows etc have been fitted.
I can't see that really working for self employed window cleaning canvassers/sales bods though as the profit margins are very small.
After saying that if going down the commission only route you would have to pay them commission x-times the value of cleans to make it worth their while. Then maybe on top an hourly rate to keep them interested as door knocking can be sole destroying at the best of times.
Finding a good self employed canvasser/sale pearson willing to work for you only 8 hours a week that is not going to try ripping you off and/or cause you load of grief will defo be a chalange for sure.
Good luck with that one