If you have informed the customers that your coming to do there windows and make sure you have access to the back, I don’t think there’s any reason to let the customer get clever with you.
Bill them the full amount, they soon learn if they don’t and complain drop them for being PITA customers.
I know what you're saying but in all fairness splitting my ladders and climbing over a 6ft gate doesn't fuss me that much. Also if I sacked every PITA customer I'd be left with about as many customers as pork chops at a Jewish wedding.
Don't get me wrong! I like the easy life and if somebody becomes a real PITA they get canned, but as in all industry "perfect" customers are few and far between and so us providers have to accept the foibles of the minority.
Shopkeepers have shop-lifters
Chefs have fussy diners
Taxi drivers have p1$$ed people being sick in the back of the cab
Electricians get electrocuted
Actresses occassionally have to appear nude...and us window cleaners?,..
...well occassionally the customer forgets to unlock the gate. If I can rectify the problem by nipping over the fence/gate then why not?
And as for charging people for work not done I don't think you can legally do that!