What determines if a package can be sent as First-Class Mail?

The ability to send a package as First-Class Mail is primarily determined by its weight, specifically if it weighs 13 ounces or less. First-Class Mail is designed for lightweight items, offering reliable and timely service for letters, postcards, and small packages. The weight limit ensures that the service remains efficient and cost-effective while providing consistent delivery standards.

While size, contents, and destination may play roles in determining how packages are handled or their specific service options, the defining characteristic that classifies an item as First-Class Mail is indeed its weight threshold of 13 ounces or less, making this answer the most accurate in the context of First-Class Mail criteria.