Adding pages to your U.S. passport book is a simple process. You will need to complete Form DS-4085 and mail it and your passport to the address given on the form. You do not have to pay for the extra pages.
It will take four to six weeks to receive your passport and extra pages from the Department of State. If you need your passport and pages more quickly, you may wish to pay for expedited service. The fee for this service is $60. You should receive your passport in two to three weeks. Remember to mark the outside of your mailing envelope with the word "EXPEDITE" in large letters. You may also wish to pay the $14.96 fee to have your passport and extra pages sent back to you via overnight delivery.
In either case, the Department of State recommends that you send your passport and Form DS-4085 via a traceable delivery method. Paying the U.S. Postal Service for delivery confirmation is one option; overnight delivery by the Postal Service or Federal Express is another.
The Department of State also suggests that you use a secure envelope (Tyvek or an equivalent) to protect your passport while it is en route to the National Passport Processing Center.