To be fair your standards are obviously high and there's not much option other than elbow effort and a cloth when it comes to UPVC. Your probably giving away a part UPVC valet for free with your first clean, as you know it will stay that way to.
Might be worth selling that to the client as part of the first clean, hence more money

Or at least promoting it as a free service that normally costs. Then your seen to be giving them something for nothing, the fact that you do it anyway means there's no extra cost to you.
Would be super handy to develop a process that didn't require a ladder though, I have quite a few chemicals to test, snow foams etc. They might work

But you'd have to go some to beat the usual method.