Subject: An apology from [Company]

Dear [Customer name],

I'm writing to apologise for [what happened — specific].

This shouldn't have happened. The cause was [specific — not a vague 'human error']. The responsibility is ours, not yours.

To put it right, we're doing the following:

1. [Immediate fix — what we're doing now]
2. [Remediation — what we're giving back / refunding / replacing]
3. [Prevention — what we're changing so this doesn't happen again]

You don't need to do anything for the above to happen.

If you'd like to talk this through directly, I'm available on [phone / email] — please reach me directly rather than the main support channel.

Thank you for your patience while we make this right.

[Name, role — most senior person whose accountability this falls under]