Today we duly received our order (white sun hats: our PO #J14) for which we thank you. However, we are afraid/ we regret that three of them are badly stained,
badlyには、この汚れ分は商品として使い物にならない、という含みを表現します。
仮に、ちょっとした汚れていないくて、badlyと書くと効果的です。
and we cannot supply them to our esteemed customers. We would like to ask you to replace them with new ones with the freight prepaid /paid on your side.
Thanks for your call, and we are sincerely sorry for the inconvenience we happened to cause to you.
*happen to ということで、今回、たまたま偶然にミスヲしてしまった、という感じを出します
いつもはミスなどありませんよ、ということを含ませています。
以下、対策などを、箇条書き的に書きます。
First, we’re going to dispatch the three new hats to you today.
Second, we’re going to check if our outgoing inspection system does really work and what happened when we checked your order.
As soon as we find out any causes of this problem, we will surely let you know and take proper preventive measures. Again, we are sincerely sorry for this matter.
↑の最後の as soon as--からの文章は、必ずつけるべき情報です。 そして、実際に、原因究明をし対策を講じる動きが大切です。