A letter of apology to customer regarding poor service performance. It explains the reason was due to inexperienced sales representative and vows to address the issue.
We apologise for the awful circumstances involved and will investigate the matter further. Pardon us for the repetition, we understand the issues are:
[[Name] is a new representative and was probably not aware of his behaviour is causing issues within your [company / family].] / [Please provide other reasons if any.] [We will be providing further sales training to all our sales representative to address this issue.] [Also, we will be making a note of this incident to be discussed in our next management meeting.]