When is a commercial letter actually worth sending?

A commercial letter earns its place in one of three ways, and it is worth sending whenever any of them applies. First, when a contractual right depends on it: many entitlements, to time, to money, to reserve a position, exist only if you notify in writing within a window, so the letter is not a courtesy but the thing that keeps the right alive. Second, when a fact will be worth proving later: a verbal instruction confirmed the same day, a delaying event recorded when it happened, a condition noted before it was changed, all turn a memory into evidence while it is cheap to create. Third, when silence would read as agreement: an unanswered cut, an unchallenged assertion, a course of dealing that hardens over months, all of which a short factual note quietly interrupts. The register matters as much as the timing. The useful letter is flat and precise: it states what happened, cites the clause, and stops. The temptation, once something has gone wrong, is to argue the whole case in one email, which overstates, concedes points by accident and reads later like the opening shot of a dispute. If a letter would help you months from now when nobody remembers the conversation, send it now, calmly, while writing it is still routine; the letters that do damage are the ones written late and hot rather than early and cool.

Take a roofing subcontractor told on site to change a detail by the contractor's foreman, who does the work and says nothing in writing because the relationship is good. Four months later the account is being settled, the foreman has moved on, and the contractor's surveyor has no record of any instruction, so the extra work is treated as original scope and not paid. Contrast the same instruction handled with a two-line email the same afternoon: this confirms I was asked today by the foreman to change the ridge detail as discussed, which I am treating as a variation and will price. Nothing about that email is aggressive, and at the time it feels almost unnecessary. But it is the difference between a variation simply valued and paid and one that cannot be proved to have been instructed at all. The letter worth sending was the calm one sent early, not the angry one otherwise sent late.