What service allows customers to send items with guaranteed delivery?

The service that allows customers to send items with guaranteed delivery is Priority Mail Express. This offering from the United States Postal Service provides expedited shipping with a promise of delivery by a specific time, typically overnight. Additionally, it includes tracking and a money-back guarantee if the package does not arrive on time. This makes it the fastest option available, ideal for urgent shipments.

Other services do not offer the same level of guaranteed delivery. For instance, Standard Mail typically focuses on bulk quantities with delivery times that can vary and no delivery guarantee, making it unsuitable for urgent needs. Priority Mail provides expedited service but does not guarantee specific delivery times, as it generally aims for delivery within one to three business days. Parcel Post, primarily used for shipping large items, also lacks a delivery guarantee and can take longer than the other options. Thus, Priority Mail Express stands out as the only service that ensures guaranteed delivery.