Stay calm and ask for their side
Let them explain before you defend the fee. Many disputes are a real emergency or a missed reminder.
Check the record
Look at the booking, the reminders you sent and when they cancelled, if they did.
Restate the policy plainly
Quote the rule they agreed to at booking, in one or two sentences, without sounding cold.
Decide to uphold or waive
Waive when the reason is genuine or your reminder failed. Uphold when the policy was clear and the notice was missing.
Offer something in return
If you uphold, offer a fair rebook. If you waive, say it is a one-off.
Fix reminders
If several disputes mention reminders, send them earlier or by a second channel.