Lead with the new date
Put the new estimated ship date in the first line.
Say why in one sentence
Give the real reason, such as a supplier delay, without excuses.
Offer a clear choice
Let them wait, swap for a similar item or cancel for a full refund.
Keep the rest of the order moving
Say whether other items will ship now or together.
Promise an update
Tell them when you will write next, and do it.
Make cancelling easy
Include the cancel link or reply instruction in the same note.