Short sentences, ordinary words
Your reader is on a phone, mildly annoyed, and skimming. “We have refunded it” beats “we have processed the requisite reimbursement” in every measurable way.
No false cheerfulness
Exclamation marks on a complaint read as mockery. Match the seriousness of the situation; warmth is conveyed by being useful, not by being bubbly.
Say who is doing what
“It will be looked into” is a sentence with nobody in it. “I have sent this to our engineer, and I will come back to you by Thursday” is a commitment.
End with the next step
Every reply should close by saying what happens now and who does it. Ambiguity at the end is what generates the follow-up ticket.