Be polite. Even if you are writing with a complaint or concern, you can be polite. Consider the recipient's position and offer to do whatever you can, within reason, to be accommodating and helpful.
For example, an impolite complaint might read: “I think your sunglasses suck and I am never buying them again.” A polite complaint might read: “I am disappointed with the construction of your sunglasses, and I plan to take my business elsewhere in the future.”