I would just be honest with them.
Explain all the reasons why this job would be so much hassle and that to do the job would cost them a fortune. Ask if they have any young nephew's, grandchildren etc.. they could get to do it one weekend.
I know they are a lovely couple an all that but if your doing the job for less money than if you were cleaning other peoples windows, well who's laughing at who??
Sometimes you just have to be tough on yourself.
If all else fails why not offer to do the work when you have time or if it rains?
Decisions, decisions