Making complaints and replying to letters of complaint

For making complaints

Opening the letter
I am writing to complain about ...
I am sorry to tell you that ...
I am writing with reference to ...
Eplaining why you are dissatisfied
There seems to have been an error
There appears to be a mistake / a misunderstanding ("Оказывается ..." можно юзать когда глобалы сделали что-то неправильно)
Requesting action
I would be glad if you would look into the matter
Suggestion action
The best solution would be to ...
I'd like to suggest that ...

For replying to letters of complaint (or Letters of Apology)

Showing understanding and sympathy
I am very sorry to hear that ...
I am sorry that you were dissatisfied with ...
Apologising
I would like to aplogise for the inconvinience you suffered (the problems you experienced).
Please accept our sincere apologies
Indicating action
I have looked into the matter
I have checked with the staff involved
Offering compensation
To compensate you for the inconvinience, we would like to offer you ...
Ending the letter
Thank for bringing this matter to my attention.
Once again, my apologies for the inconvinience caused.
We very much regret the embarrassment (затруднение) / annoyance (досада, раздражение) you experienced.
I very much hope that you will continue to use our company in the future.

Useful expressions from examples
I would like to apolgize on behalf of Alpha company ... (от лица)
I showed them your letter and explaned how upset (огорчение) you were.
They now regret that they did not take corrective action