He rushed into the office, flourishing a letter of complaint.

他急冲冲地闯进办公室, 手里挥动着一封投诉信.

He stormed into the office, flourishing a letter of complaint.

他挥舞着一封投诉信冲进了办公室。

I write a letter of complaint if the product or service is not what I expect.

如果产品或服务达不到我期望的,我会写一封投诉信.

In a letter of complaint you will be looking for a specific result, such as a refund.

写投诉信则旨在寻求结果，比如要求退款。