The maximum thickness for letters sent via Express Mail is established at 1/4 inch. This measurement is important because it helps determine the eligibility of a piece of mail for Express Mail service, which is designed to provide expedited delivery. When preparing letters for this service, it's essential to adhere to the specific dimensions outlined by USPS, including the weight and thickness, to ensure that the mailpiece is processed correctly and can achieve the promised delivery speed.
Understanding these specifications is crucial for retail clerks, as it aids in providing accurate information to customers and maintaining compliance with postal regulations. Express Mail, now known as Priority Mail Express, offers guaranteed delivery, and adhering to the thickness requirements contributes to the efficiency of mail processing and delivery.