I bet all the contractors asked them if they had a window cleaner - window cleaners just love cleaning up all the mess we make - after all it's what you pay them for. When you refuse they will throw a tantrum.
If you have problem with payment now, wait until you quote them for the standard conservatory clean. Large conservatories with lots of glass take longer than the house to clean, but they won't see that.
Good advise, don't quote, refuse job and walk away if you feel pressurised.
It's their right to ask you to quote for the job and it's your right to be able to refuse.
Spruce