In 44 cruises we have only had to write two letters of complaint. Both were done on board and handled on board.
The first was a specific shore excursions which had a specific list of what would be seen. When we took the excursion, it did not include one of the specific elements. We wrote to the shore excursion office on board (along with quite a few other people, apparently) and were promptly credited 1/3 of the cost of the excursion.
The other was an incident on boarding in Istanbul when my wife and her friend, along with another couple, were treated very rudely and inconsiderately. My wife wrote a complaint and got back a letter of "apology" which was actually a letter of justification. She was not very happy. Then later she ran into the other couple, and learned that they had gotten a plate of chocolate covered strawberries with their apology! The letter she wrote then did NOT follow Paul's guidelines! I am afraid it was more than a bit STRONG! Next day we got our strawberries too.