Ask them to remove all furniture, and carpet before cleaning, a plastic sheet with a ladder on is asking for trouble, price it at double your hourly rate and say it is timed, this way you dont lose out on money, if they ask for a fixed price triple the time you think it will take, con roofs on the inside are a pain in the backside, i got caught out once "never again" step ladder and trad I would say, then again never tried wfp on it, if tiled floor sheet it with plasic as the grout with get stained and re grouting as a royal pain in the a** been there before.
Dont ever avoid things that make good money, this only says you dont know how to do it correctly. A hard job will always be higher in price.