Please refer to the attached file in this email for your right invoice.
If you're not sure, an apology offers you the chance to "own" mistakes you made, but re-establish what you think was okay.
It does not add a reason or a cause for the wrong.
Thank the client for patience and remind them that you value their loyalty.
You will be proved right in what you say, and your judgment against me is just.
Give explanations for the delay.