Which type of trays must always contain bundled mailpieces?

The correct answer is related to the handling and sorting requirements of mail within the postal system. Trays designated for card-sized pieces must always contain bundled mailpieces to ensure efficient processing and delivery. This is primarily due to the smaller dimensions of card-sized mail, which can be easily mixed with other types of mail if not properly contained and bundled.

Bundling these pieces together helps maintain organization within the tray and minimizes the risk of damage or misplacement during handling. It also allows postal workers to quickly identify the contents and ensures that all pieces are processed uniformly, which is crucial for maintaining delivery timelines and efficiency.

In contrast, the other types of trays mentioned may not have the same stringent requirement for bundling. For instance, oversized pieces, flat envelopes, and letter-sized pieces have different standards based on size, volume, or specific processing needs that allow for a varied approach in organization.