Not giving the opportunity for a refund to a customer when shipment has been delayed is against some FTC rule:

Selling on the Internet: Prompt Delivery Rules

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By law, you must have a reasonable basis for stating that a product can be shipped within a certain time. If your advertising doesn't clearly and prominently state the shipment period, you must have a reasonable basis for believing that you can ship within 30 days.

If you can't ship within the promised time (or within 30 days if you made no promise), you must notify the customer of the delay, provide a revised shipment date and explain his right to cancel and get a full and prompt refund.

For definite delays of up to 30 days, you may treat the customer's silence as agreeing to the delay. But for longer or indefinite delays - and second and subsequent delays - you must get the customer's written, electronic or verbal consent to the delay. If the customer doesn't give you his okay, you must promptly refund all the money the customer paid you without being asked by the customer.

Finally, you have the right to cancel orders that you can't fill in a timely manner, but you must promptly notify the customer of your decision and make a prompt refund.

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