When you cannot supply.
THE OPTIONS
Hide the product Show it as out of stock Allow backorders Take names for notification
WHAT TO AVOID
Leaving it orderable.
An order you cannot fulfil is worse than a lost sale.
SHOWING OUT OF STOCK
Better than hiding, usually.
The customer sees you stock it and may return.
BACKORDERS
Only if you can state when it will ship.
"Ships in two weeks" is acceptable. Silence is not.
NOTIFICATION SIGNUP
Capture interest while you restock.
Useful, and only if you actually notify them.
FOR PERMANENTLY DISCONTINUED ITEMS
Do not simply delete the page.
Keep it, mark it discontinued, and link to alternatives.
Deleting loses search value and produces not-found errors for existing links.
WHAT TO TELL CUSTOMERS WHO ORDERED
Immediately. Offer an alternative, a wait, or a refund.
Silence produces disputes.