80% of the sales are made by 10% of the salesmen on the 5th call.
I'd be sending out letters as part of that premise. Sure, what you send today may have a minimal effect as far as a direct response immediately but it would be a part of the process of getting them on board. A response to a mailing campaign that is sent out only once is lower than one that is sent out over a few weeks/months so you have to start somewhere.
A part of a successful mailing is branding and that would be part of what that first mailing would be doing for you.
If it is me, I'm sending out the letters right now (who else is doing that this time of year for ORM in your area, I bet nobody so you have no competition in that respect) and following up with another mailing after the Holidays. In that mailing I would be letting them know that you mailed them before as well as pitching them.