We were very sorry to learn that …
We regret to learn that our consignment has not reached you in good order.
We are very sorry that our consignment did not meet with your expectations.
We regret our mistake in expediting your order to the above address.
We do, of course, accept that you are entirely justified in making your complaint.
It seems to us that your complaint is entirely / partly justified.
We will immediately substitute the goods with models of the same standard.
We will immediately replace the goods by similar ones of the same quality.
If you decide to keep this consignment, we will gladly offer a 10% discount.
We apologise sincerely for this unfortunate affair.
We assure you that we will do our utmost to continue to provide the service you have come to expect.
We will do whatever is in our power to …
We hope that you will understand our situation and trust that you have not been unduly inconvenienced.