ご連絡いただきまして誠にありがとうございます。
この度は商品が届いていないとのことでご迷惑をおかけしまして申し訳ございません。
日本の郵便局に確認しましたところ、現地アメリカの郵便局で商品が届いていないことを調査することが可能とのことでした。
つきましては、大変お手数ではございますが、アメリカの郵便局から「荷物不着の調査依頼書」という書類が送られてきますので、そちらに署名いただき返送用の封筒に入れてポストに投函いただければと思います。
ご不便をおかけしますが何卒よろしくお願い致します
I am sorry for having caused you a lot of inconvenience because the item has not been delivered.
We confirmed with the post office in Japan, and they told that it is possible for the local post office to investigate whether the item arrived in the US or not.
Therefore, I am sorry for bothering you, but I would like you to put your signature on the document "the request form for investigating the delivery failure of the item", to put it in the envelope and to post it when they are sent to you from the post office in the US.
I am really sorry for causing you an inconvenience, but I appreciate your help.
We apologize for the inconvenience that has been caused as the item has not been delivered to you for this occasion.
As we confirmed with the Japan Post, it is possible for them to investigate an undelivered item at the local USPS.
We are very sorry for the trouble, but would you please sign and return with an enclosed return envelope as a document called "Missing Mail Search Request" from USPS.
We are so sorry about the inconvenience and thank you for your cooperation in advance.
We apologize for the item has not arrived yet.
As I checked at the Japan Post Office, it is possible to check the item not arrived in America.
Therefore, I am sorry to take your time, as there is a document "for request to investigate the item not arrived" sent from American Post Office, Please sign it and post in a return envelope.
We are sorry for this inconvenience.
すみません。いくつか単語を訳し落としました。
2文目を "I am sorry for having caused you a lot of inconvenience because the item has not been delivered this time."とします。また、最後から2番目の文を、
"Therefore, I am sorry for bothering you, but I would like you to put your signature on the document "the request form for investigating the delivery failure of the item", to put it in the return envelope and to post it when they are sent to you from the post office in the US." と差し替えてくださいませ。よろしくお願いいたします。