I always get my customers to sign a survey / quote form , on this it confirms the work to carried out and price, which the customer signs before I start the work , this helps to avoid any confusion about what I am cleaning and the price, if anything is changed I amend the form and get the customer to re sign and time + date ,
On the form is H&S advice re slip hazard etc, The customer gets a copy of this form and we keep a copy on file .