When it comes to the windows getting sprayed. I give all the windows a clean, squeegie etc. even the ones that didn`t get sprayed, after the drive has been done.
A lot of the time the customers are chuffed that they have had a free window clean and this has led to some window related repeat custom.
As far as flower beds are concerned I use sheets of thin, strong plastic that are good as they are light weight.
If you feel that the drive owners are watching your every move for fear of damaging their drive, before you start give them a good demo. If there are some rough parts, point them out first.
I am just starting out in this line of work and had the same worries at first aswell, but as long as you show the customers what you are using, or doing they seem to relax a little more.
Best of luck.