If you wont to cover your self you could set a terms & conditions of doing business with commercial customers, what we do thou is put the quote in the post and at the bottom get them to sign saying they agree for us to processed with the work at the set price and times of visits, if anything did happen and say they didnt pay you and you had to take them to a small claims court, then the judge can see they agreed for you to do the work, which is better than, no agreement in place
Andy