What types of items must be mailed using First-Class Mail or upgraded to Priority Mail Express?

Handwritten or typewritten personal correspondence must be mailed using First-Class Mail or upgraded to Priority Mail Express because these services are designed specifically for personal letters, including cards, small parcels, and other time-sensitive communications. First-Class Mail is the most efficient and economical way to send personal letters and correspondence, providing reliable and timely delivery.

This particular category of mail, such as personal letters, benefits from the quicker delivery times and the tracking options available with Priority Mail Express for those who require even faster service. The other types of items listed, such as promotional flyers and catalogs, have alternative mailing options that may not require the same level of service as personal correspondence, making those items less urgent in terms of delivery speed.