I'll quite happily say ok and leave it until next time, as long as they don't make habit of it.
Once or twice a year, no problem, it's not like many do it.
Twice in a few months and I still say ok, but they don't see me again.
I wouldn't!
Are you telling me that you'd drive upto a house, unreel your hose, and just before you're about to start work, your customer sticks their head out of the front door and says, 'Not this month, mate'? So then you've got to pack up and move on.
And then you'd return to clean them again?
No way... no ruddy way.
I've had ONE 'Not this month, mate' this year, and I dropped them. It rarely happens to me, because I've always binned the customers who've tried this.
I've even binned a £100 domestic account (which Ian_Giles does now) because she did the same to me (but Ian and I did a bit of 'price discussion' on that one, so she's paying even more now. That'll teach 'er

If you agree terms and conditions, stick to them; don't let your customers treat you like 'rubbish', or you'll end up with a naff load of work.
In previous years, some customers have approached 'Wor Lass', thinking she'd be a 'softer touch' than me, and when they've said, 'Can you leave us this month?', unless they've had a good reason, Wor Lass says, 'So you don't want your windows cleaned by us anymore?'
They've always relented; this is one of my pet hates.
Be firm with your customers when they're being unfair to you.