I have to mail the W7 + 1040 To IRS POST BOX NUMBER.. As it's a post box number can I use USPS certified mail to send the documents as on USPS website it states that

When you use Certified Mail, you receive a receipt stamped with the date of mailing. A unique article number allows you to verify delivery online. As an additional security feature, the recipient’s signature is obtained at the time of delivery and a record is maintained by the Post Office

As it's a po box number no body is going to sign there

Internal Revenue Service
ITIN Operation
P.O. Box 149342
Austin, TX 78714-9342

A few years ago I was told that customers that receive large volumes of incoming mail were allowed to bring back the signed return receipts later!

On another occasion a carrier gave a "restricted delivery" item to the person who answered the door without even obtaining a signature. The person answering the door was not the person the item was addressed to.

Since then I've been purchasing a certificate of mailing for most things. Only $1.10 compared to $2.70 for certified http://www.usps.com/send/waystosendmail/extraservices/certificateofmailingservice.htm
USPS - Extra Services Prices (http://www.usps.com/prices/extra-services-prices.htm)

Actually, someone from the IRS will sign for the document, as the IRS gets certified mail with return receipt on numerous occasions.