What is the primary characteristic of Express Mail?

The primary characteristic of Express Mail is that it is the fastest way to send mail, allowing for timely deliveries of packages weighing up to 70 pounds. This service is designed to provide expedited shipping options, typically offering next-day or specific time-definite delivery, making it ideal for urgent parcels. Express Mail stands out due to its reliability and speed, ensuring that items are delivered as quickly as possible when compared to other mailing options.

The other choices do not accurately describe the main features of Express Mail. For example, while it may not be available on certain holidays, it is generally offered throughout the week, including weekends in some locations. Additionally, Express Mail is not limited to heavy packages; it can accommodate lightweight items as well. Lastly, the requirement for scanning packages is a part of the service to track the items and ensure proper handling, contradicting the option suggesting that no scanning is involved. Thus, the fastest shipping capability is what fundamentally defines Express Mail.