Dear...
Reasons for writing - I am writing to you regarding.. / I am contacting you about.. / I would like to bring your attention to.. /

Something good - Firstly, let me say I am a fan of.... / I am a regular customer / I am a loyal customer of yours / I have used your … many times /

The problem with details and examples if needed -  that said / However, / With that being said / Having said that I / Unfortunately I have… What you think should be done - In my opinion I think… / I think it is only fair / I think as this has happened, you should..

Signing off - I look forward to hearing from you / I hope to hear from you soon / I look forward to your reply/response / I would appreciate a fast response on this matter /

Yours faithfully/yours sincerely
Name

Note we use yours faithfully when we don't know the name of the person we are writing to

Replying to complaint

Dear

Thanks for contacting - Thank you for your recent email/letter/phone call / We really appreciate you getting in contact / Thank you for your feedback

Apologise from the problem(s) -  I am awfully sorry about.. / It Is unacceptable that you… / Answering any questions and what you are doing about the problem -  We are going to … / We will .. right away…

Offer alternative/solution - We would like to offer you … / In the meantime… / To apologise we would like to… /

Signing off - I look forward to hearing from you / I await your reply / I hope this meets your expectations

Yours sincerely
Name