Use a real name and a human voice
Sign with a person's first name and avoid copied wording. People can tell a template at once.
Move the details to a private channel
Ask for the order or account details by direct message or email, never in the open. Keep the public part to an apology and a next step.
Fix it and then report back
After you resolve the problem, come back to the public thread with a short update. A visible resolution is worth more than a quiet one.
Do not argue or delete
Disagree politely if the facts are wrong, but never start a fight or remove honest criticism. Deleting posts usually makes things worse.