While letters take a lot of time and require letterhead and envelopes that should look professional, they can be used on a limited basis to reach a niche market of potential customers similar to one or more of your existing customers.
The similarities among commercial customers or residential customers could present a marketing opportunity. I experienced an example of that with lecterns (podiums) for hotels. One of the large local hotels called me to build a small, table top lectern. I designed one and built it for them and they were happy with it.
I realized that there were several large hotels in the city so I prepared a brief sales letter with a picture of the lectern that I had built. I found the address of the ten largest hotels in the city and sent the letter to the Director of Engineering at each location. Two weeks later I had several thousand dollars worth of orders for lecterns.
A couple of years later one of those hotels called me back because the lecterns were getting banged up badly and they wanted to know if I could design one that would hold up better to the rough handling they received and could more easily be repaired. So, I came up with a design that I will be sharing in a future post and sold them eight standup lecterns. I sold a dozen of them to a hotel in a community about 80 miles away and three more to a hotel in another state.
This was an enormous return for preparing and mailing only ten letters. Obviously, if one hotel can use what you made, others can probably use it also. The same goes for other businesses and homes.
-- Bill, Austin, Texas, http://woodworking-business.com