The pre-delivery request is from the delivery agency not from Apple nor ATT. You can basically use a notepad and write down the following. There is no official note/letter. Also, it is up to the carrier to accept your note. He/she has the right to release even with a signature sheet if he/she thinks it is unsafe to leave package.
To delivery carrier, please leave my package in front of my door. Here is my tracking number and signature. I will take full reponsibile if it is stolen or missing from my front door.
This is wrong. The person/entity sending the package, not FedEx, UPS or DHL, determines whether a signature is required and the sender can also specify whether a signature waiver can be used. It costs more to send a package with the 'Signature Required' option since it takes more time for the delivery person and they have to track and store this information.