How to write a transactional letter?
A 'transactional letter' has specific purpose and a result. For example, a letter of invitation, a letter of complaint and others that require a reply or subsequent actions.
In the exam you will have a description of situation - you might need to reply to a letter or e-mail or write a letter based on some notes or other information. Things you should take into account before writing a transactional letter:
ViewpointTry to think yourself into the position of the writer and write from this point of view.
ParagraphingWork out a paragraph plan before you start writing. Each paragraph should have a specific purpose.
ParagraphingEach paragraph should have a clear topic and purpose. The first sentence should say why you are writing, the final sentence should make clear to the reader what you expect to happen next.
Key factsInclude all the factual information provided in the question. Use your own words.
StyleChoose an appropriate style, remembering what the purpose of the letter is. For a less formal letter, start with Dear + name and end with Yours sincerely. For a formal letter, start with Dear Sir or Madam and end with Yours faithfully (if you don't now the name).
Rules for Writing Formal Letters in English
1) Your Address:
The return address should be written in the top right-hand corner of the letter.
2) The Address of the person you are writing to:
The inside address should be written on the left, starting below your address.
Different people put the date on different sides of the page. You can write this on the right or the left on the line after the address you are writing to. Write the month as a word.

Salutation or greeting:

1) Dear Sir or Madam,
If you do not know the name of the person you are writing to, use this. It is always advisable to try to find out a name.
2) Dear Mr Jenkins,
If you know the name, use the title (Mr, Mrs, Miss or Ms, Dr, etc.) and the surname only. If you are writing to a woman and do not know if she uses Mrs or Miss, you can use Ms, which is for married and single women.
Ending a letter:
1) Yours faithfully
 If you do not know the name of the person, end the letter this way.
2) Yours sincerely
If you know the name of the person, end the letter this way.
3) Your signature
Sign your name, then print it underneath the signature. If you think the person you are writing to might not know whether you are male of female, put you title in brackets after your name.
Content of a Formal Letter
First paragraph: The first paragraph should be short and state the purpose of the letter- to make an enquiry, complain, request something, etc.
The paragraph or paragraphs in the middle of the letter should contain the relevant information behind the writing of the letter. 
Last Paragraph: The last paragraph of a formal letter should state what action you expect the recipient to take- to refund, send you information, etc.
Useful expressions:
I am writing to complain about...
I am sorry to inform you that...
I was very disappointed with...
I have some complaints about...

I am writing for information about...
I would like to learn/ know more about...
I would like to ask whether/ if...
I would be grateful if you could...

Asking for particular actions:
I would like to ask you for...
I would suggest that you/ your company...
I think that I can ask for some compensation.
In the light of the above, I would like to ask you for...

Letter of complaint
Q: You invited your friend to a fancy restaurant. You found their advertisement in the local newspaper and it looked convincing. Unfortunately, the place did not meet your expectations and you were disappointed. Write a letter to Mrs Talker, the manager of the restaurant, and ask for some money back.
Restaurant 'Romantica' is the best place you can meet your friend and spend a nice evening in a good company. The place is designed to ensure lots of privacy. Comfortable armchairs and quiet soothing music in the background make the place perfect for those tired of everyday rush. Discreet service is our first priority. Our menu is created by people who worked for the best restaurants in the world. Great choice of vegetarian dishes. Come and enjoy! Satisfaction guaranteed!
32 Hight Street
CT19 5JY
Mrs Elizabeth Talker
33 The Glades
CT20 2PR
10 May 2010
Dear Mrs Talker,
I am writing to inform you that I was very disappointed with my visit to your restaurant two days ago. I read your advertisement in the newspaper and decided to spend an evening there with my friend. Unfortunately, I have a couple of complaints concerning this visit.
According to your advertisement the place is perfect for having private conversations in relaxing atmosphere. However, it turned out that the music was so loud that I could hardly concentrate on the chat with my friend. The waiter asked to turn the music down shrugged his shoulders and said that other guests liked it that way. Is that what you mean by 'discreet service'?
To make matters worse, the place was crowded and waiting for an ordered meal was intolerably long. I must admit that the cuisine was quite good but the prices were way too high.

I would like you to take my points under consideration and I expect a refund from your restaurant. I trust you will give this matter immediate attention. I look forward to receiving a reply from you.
Yours sincerely,
John Smith