To write a letter of complaint you need to know at first that it is a formal letter. That means that the xoxo in the end won't be acceptable. So just.... just don't send kisses to someone you are going to write a rough complaint. Remember to use the formal language. "Hi Smith, the computers you are producing suck" is not a good idea to send to anyone. It won't fix the problem you are dealing with. The most important thing is to clearly know what do you want to say and who will be the one to read this.
The basics that you have to know are:
1) remember about paragraphs - without them it looks like a bunch of words not a letter.
2) focus on what you want to say - nobody really cares that you bought something in five different stores, the reader just wants to know why his product was so bad to write a letter about.
3) don't you dare using the informal language - you are not writing to a friend!
4) remember that if you begin with - Dear mr. Smith at the end you are obligated to put Sincerly XYZ, but if you decide to write Dear Sir the only option you can choose is Yours faithfully - at least that's what every teacher would say.
To sort everything out here you have the example of how the letter of complaint should look like.:
Dear Mr. Smith,
I'm writing to You in order to explain few things that caused my dissatisfaction after finishing the "Mai Dan - speak mandarinian like in China" course.
First of all I would like to say how excited I was signing up for this course provided by your skilled team of native speakers. The brouchure said that for 3500zlotys the participants are going to get five hours with native speakers, two hours with teachers focusing on grammar, three hours learning the alphabet and vocabulary, per week obviously. It should be 10 hours weekly and two, free, additiona ones for language practice. It seemed to me that after a month my chinese maybe wouldn't be fluent but, at least, I would be able to hold a proper conversation in this language.
Unfortunately none of what was said in the brouchure turned out to be true. The main disadvantage is that the native speakers were japanese and it was hard to notice if they were speaking mandarinian or their real native language, because they were mixing them.
Secondly the amount of hours didn't cover the 10 hours per week as it was said. In the half of the course our natives flew back to Japan and new teachers were from Poland.
I was told that in the price we will get the books and everything else needed. What I got is a free pen. For books and special notebooks I had to pay extra. Since the end of the course I'm still waiting for the package. ladies form the office told me that it is comming form China so it may take a bit longer. Maybe after few months I will finally get them.
Last but not least I'm not able to say anything more in mandarinian than "check please" but after two weeks with japanese natives I know basics of their language.
I advise You to look for real Chinese natives and start to take care of Your clients because Your brouchure says one thing and it turns out not to be that great on the course. Otherwise you will end up without any customers.