The essential rule in writing complaints

The essential rule in writing a complaint letter is to maintain your poise and diplomacy, no matter how justified your gripe is. Avoid making the recipient an adversary. It is no use writing complaints in an angry, abusive tone. A reference to the previously satisfactory deliveries and the high standing of the partner will in most cases prove more helpful.

What grammar structures are preferable?

Do not use sentences like:	Use the passive and impersonal structures:
· “You must correct your mistake as soon as possible.” · “You made an error on the statement.” · “You don't understand the terms of discount. We told you to deduct discount from net prices, not c. i. f. prices.”	· “The mistake must be corrected as soon as possible.” · “There appears to be an error on the statement.” · “There seems to be some misunderstanding regarding terms of discount. Discount is deducted from net prices, not c.i.f. prices.”