overnight mail from post office Overnight mail from the post office is typically available through USPS’s Priority Mail Express service, which guarantees next-day delivery. This service provides an expedited shipping option for both domestic and international shipments, ensuring that packages, documents, and letters reach their destination quickly. USPS Priority Mail Express offers 24-hour delivery to most locations, including weekends and holidays, and comes with tracking, insurance, and a money-back guarantee if the delivery is late. It’s ideal for time-sensitive shipments that need to be delivered fast.