Dave,
Had you been a more unscrupulous sort of person, had you slipped and hurt yourself, you could've sued the customer.
I read not so long ago that a Sommerfield shopping delivery man sued a customer because he slipped on their property and he hurt himself. I believe there is some law, or act, which says that people have to take all reasonable precautions to ensure their property is safe for tradesmen.
If someone could expand; or tell me (us) if I'm right or wrong here; I'd be interested.
I personally think your customers are being unfair; but I would just put their prices up as has been suggested; and keep putting them up each year.