Non-machinable surcharge will be applied to first class letters when the item is:

The application of a non-machinable surcharge to first-class letters primarily relates to how the item is packaged. When an item is in poly bags or poly wrapped, it presents challenges for automated sorting machines, which are designed to handle standard letter sizes and shapes. Such packaging can cause jams or delays in the processing system, leading to additional handling and processing requirements.

In contrast, while thickness, size, and fragility might influence mailing costs or handling considerations, they do not specifically relate to the criteria that trigger the non-machinable surcharge like the type of packaging does. Poly bags or poly wrapping disrupt the automation flow, resulting in extra fees for manual processing. This is why selecting the option related to poly bags or poly wrapped items reflects the correct understanding of the criteria for the non-machinable surcharge.