Write an email to a customer with an apology.

Dear [Name],
I apologize for the inconvenience caused.
user
Dear [Name], 
I am sorry for the inconvenience caused due to the issue you faced when trying to use our service. We understand how important it is for you to get the expected results as quickly as possible and we deeply apologize for not meeting your expectations. 

To make it right, we offer you [offer]. Please don't hesitate to contact us if you need any further assistance. 

Sincerely, 
[Your name]
assistant