Can you find out the names of the managers of the other outlets. Than at least you have someone to address the letter to.
A good idea is to find out their phone numbers to for follow up.
I would think along the lines of:
Dear Mr/Mrs X
I currently provide the window cleaning service for Mr/Mrs X of XYZ outlet.
I am now in a position to offer the same service to your outlet and can provide a free no- obligation quotation.
you can put in details of how you work and regularities etc etcbut keep it to the point!
Finish off with something like:
Please feel free to contact me with regard to the above, although I shall contact you in the very near future.
Kind Regards
Something like that,best if you put it on your own words as this then does a certain amount of personalisation to it

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