If you a make your application in person at the Embassy then you are required to purchase the DX secure delivery service to deliver your passport when it is subsequently processed (this may take several days).
The DX service can be purchased from a DX representative in the lobby of the American Citizen Services on the day of your scheduled appointment.
The following prices apply
Supplementary Priority Services
All the above prices include VAT at the current UK rate Additional charges will apply for items in excess of 1kg in weight
US Embassy Terms & Conditions