To put my humble opinion to this: If I had a window cleaner, and he is a decent standard, cleans well, gets the job done with no great drama, then I'm happy.
If however, I forget to leave my back gate open and only got my front windows cleaned, then got a call or note to say "Sorry you weren't in when I called and I couldn't do the back windows as you forgot to open your gate. You will still be charged the full price though". I may be tempted to drop my WC and get the next guy from down the road or in the Yellow Pages etc, and think "Goodbye" to my EX window cleaner. Food for thought? What would you guys do?