Specifically have they asked you to quote or tender?
If its a tender is there a set process you need to follow or do they just want a price? (quote)
Have you visited the site? Can all the windows be cleaned with wfp?
Basically you need to compile a formal letter thanking them for the invitation.
Then add
* a few details about your business eg -safe contractor approved, fed member, environmentally friendly etc. basically what makes you stand out.
*Then add your preferred payment terms (be realistic not 7 or 14 days!)
*commercial references on request
*RA /MS insurance will be provided if sucessful etc
*Thank again- contact me blah blah
Now for the quote
Title (what its for)
then a short introductory passage about location what you can clean and if ther is any thing you cant. Exclusions etc
Frequency required and cost
Vat details or fully inclusive
The add a couple of quotes at differing frequencies to maybe tempt them into a more regular service
Frequency cost etc
Vat details or fully inclusive etc
Job done
I hope that makes sence, but basically you need a short to the point letter on one page then a page or two with the actual quotes on.
If you really do struggle contact me and I will see what I can do for you