You must study the tricks of the trade. That's why I'm always puzzled how few salesmen understand them 1. Don't assume nobody reads direct mail. Just as good salesmen earn a ton more than bad, good letters do a whole lot better than bad. Two years ago I saw one letter do over seven times better than another making the same proposition.
Amateurs limit letters to one side of a piece of paper.
But letters are salespeople in envelopes. You don't tell live salespeople to keep it short. You tell them to get a sale, whatever it takes. And that is persuasion. Give every sensible reason why somebody should respond and overcome every reasonable objection they may have.