How do I apologise for a late delivery in English?
Name the specific failure, say it is your fault in one short clause, describe the action already under way, and give a time when you will speak again. Put all four in the first paragraph, because the customer will not scroll for an apology.
Name the specific failure, say it is your fault in one short clause, describe the action already under way, and give a time when you will speak again. Put all four in the first paragraph, because the customer will not scroll for an apology. Keep the words factual rather than emotional: in English, a precise sentence does more work than a longer expression of regret.
Timing matters as much as wording: send the apology before the customer has to ask a third time, and put it in the first line rather than the third paragraph. In writing, one sentence of regret is enough; the rest of the reply belongs to the cause and the date. Two phrases to avoid entirely are "I apologise for any inconvenience caused" and "I am sorry you feel that way", because the first names nothing and the second moves the problem into the customer's feelings.