An email or letter of apology regarding incorrect quantity of goods delivered. The letter explains the reason for the mistake and the remedial action(s) being taken.
Thank you for letting us know that we have made a mistake in relation to the quantity of [Products] delivered. We agree with you that the fault is entirely ours and apologise for any inconvenience caused.
[When we investigated the original shipment, we were surprised to find that it does not correspond with the quantity of the order. It was purely [our clerical error] / [shipper’s error] and we have remediated the issue immediately by sending you the remaining quantity in another shipment [by air].]