Start with the outcome
Before writing the backstory, decide what you want: a replacement, refund, correction, apology or explanation. A clear request gives the recipient somewhere to go.
Build a short timeline
State what you bought or arranged, when it happened and what went wrong. Include reference numbers only in a private message to the business, never in a public post.
Explain the impact
Describe the practical effect in one or two sentences. Stick to what you observed and avoid guessing at motives.
Make the next step obvious
Finish with the resolution you want and a reasonable response date. Keep copies of the message and any reply.