Business English: the art of politeness

http://www.dorisandbertie.com/goodcopybadcopy/2011/11/01/how-to-master-the-peculiarly-english-art-of-the-polite-command/

http://www.engvid.com/polite-business-english-complain-disagree/

http://www2.elc.polyu.edu.hk/cill/eiw/adjustmentletters.htm

http://www.itrainingexpert.com/pdf/user/content/Sample-Letter-on-How-to-Reply-to-a-Complaint-Letter.pdf

http://www.asampleletter.org/complaint-response-letter.html

http://everydaylife.globalpost.com/respond-customer-complaint-writing-19091.html

http://www.blairenglish.com/exercises/emails/exercises/email_complaint/email_complaint.html

http://www.dcielts.com/ielts-letters/a-letter-of-complaint-mobile-phone/

http://englishelxna1.wordpress.com/2012/11/29/practise-complaint-letters/