This is my experience.
When I survey high end clients they tell me that they prefer to gain new services by referral only.
They don't want to let just anyone to come into their homes.
This is not to disparage just my observations throughout the years:
If the free cleaning shotgun approach works for you that's great!!
But,I wonder if anyone else that have seen the letters have the same opinion I have.
By the time the third letter comes around they are practically badgering the reader for not taking advantage of the offer.
Most people are too savvy to really think that they are going to get something for free,
(I know I get simular letters every once in a while and they go right into the trash.)
I think the marketing may turn off a lot of the recipients doing your business a disservice.
Every other year or so a company tries this approach in my area then they disappear the following year.
As a Note:
I was cleaning a whole house once except for one room.
A van pulled up they pulled out a portable.
I asked the homeowner what was going on. She said, "O' they are going to clean the free room."
six months later that company was gone.