Im not good at these sort of things and advise would be appreciated.
Dear Customer
I am please to inform you that we are expanding our window cleaning business and we're interested in providing you with professional window cleaning service for your business.
We have enclosed a free quotation attached with this letter.
Please review the quotations provided and if you are in need of our services please sign and return the appropriate quote sheet and return it in the envelope provided.
Thank you for your time, hope to hear from you soon.
Regards,
James