How about:
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GQC Window Cleaning will clean your windows at our agreed frequency throughout the year; regardless of the weather; ensuring you have a regular, fully insured and professional service from us.
However, we find it frustrating when a customer turns us away without prior notice or good reason. If this happens we shall terminate our contract.
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This might sound a bit harsh, but it definately works. I think I've only had four or five customers turn us away since we put a similair worded peice on our 'Your Windows Have Been Cleaned Chits' (that's about two years), and I think these customers didn't want to pay for a regular window cleaner anyway.
Other (good) customers jokingly commented that we must've been messed about some to put that on our chits.
We had! Twice in one day; ONCE.
I think if someone is going to try and save a few quid by saying 'Not this month, Mate', then trying to bung on an extra 25% would be a bit difficult.
But I haven't tried this, but would if it worked, but I have doubts.