The tone that works in support writing

Neither corporate nor chatty. Plain, and slightly warmer than you think necessary.

Short sentences, ordinary words

Your customer is reading on a phone, mildly annoyed. “We have refunded it” beats “we have processed the requisite reimbursement.”

No false cheerfulness

Exclamation marks on a complaint read as mockery. Match the seriousness of the situation.

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.