Check the facts before replying
Look up the order and the carrier or supplier status so your first answer is specific, not a holding line.
Say what you know and what you do not
Give the current status and when you will next update them. Do not invent a delivery date.
Apologise once and offer a fix
Offer what your policy allows: a refund, a replacement or a goodwill credit. Let the customer choose where you can.
Follow up on the date you promised
Send the update even when nothing has changed. Silence after a promise is what makes people angry.
Look for the pattern
Tag delays by cause. A repeated supplier or carrier problem needs fixing at the source.