I would never charge people if I had to ask for it, so for me, a form is essential! I used to have a separate "oops" note (funny that I used the same name as Blackcat; trying to lighten the tone, I guess). Now I just combine all extras into one form. (Additional hours; late charges; freebies--I allow each child 2 free scheduled evenings/year--and diapers, which are 50 cents each if I provide them, whether by agreement or for missing supplies.)