No, I have not written a letter exactly like this but...
I think you have already written the meat and potatoes part of it if you just clean up the part below a little bit.
Originally Posted by lazor-cut
We have finally made the decision that we are going to get out of the Residential lawn maintenance so we can focus more on High end resi landscaping and our full service Commercial accounts.
When I write anything that will take more than one sitting, I try to write down everything I am thinking then take out the parts I don't mean and anything the recipient does not need to know (or may not care to know).
Beyond that, thank them and if you have anyone to recommend offer to do so (or just do it). If I am the customer being dropped I don't think I would feel compelled to use the new company if you sold my account to them.
You have to decide how fast you want out and if there are any circumstances under which you would continue a few more mows/weeks/months. Maybe for pre-pays?
Generally, no matter how much people like your service they don't care too much about how your decision affects you, they want to know unambiguously how it affects them. I would probably close with a sentence indicating that you understand this may inconvenience them, say you are sorry about that, and thank them again.
The whole thing should be shorter than my post here. Three short paragraphs and 6-8 sentences total. Good luck with it.