What is the disadvantage of flattened mail items?

Flattened mail items can sometimes be categorized as "non-machinable," which means they do not meet specific size and shape requirements that enable them to be processed automatically by postal machinery. As a result, these items may incur additional postage fees due to the extra handling required. The classification as non-machinable is significant because postal services often charge higher rates for mail that requires manual processing or special handling, making it a key disadvantage for senders.

This understanding is essential, as it highlights how the physical characteristics of flattened mail can affect cost and efficiency in the mailing process. While other options discuss aspects like storage difficulties or special handling, they do not directly affect the postage costs tied to the classification of non-machinability.