Just a thought.
-We hire company to clean windows.
-Windows get cleaned.
-We are satisfied.
-We pay the bill.
-Window cleaning company makes profit (maybe). Employees get paid (maybe).
-We continue to use company (as long as we are satisfied).
-Company sends us Christmas card (or seasons greetings card) if we have a lot of windows and like them cleaned a lot.
Please don't make me feel guilty about not tipping the non-attendant gas pump.