Which of the following is NOT a characteristic of semi-postal stamps?

Semi-postal stamps are a unique category of postage stamps that serve both postal and fundraising purposes. The primary function of these stamps is to raise money for specific causes, often charitable in nature, which is part of their defining characteristic.

When looking at why the highest market value is not a characteristic of semi-postal stamps, it's important to consider that these stamps are typically issued with a standard postage value plus an additional amount that is designated for the cause. Therefore, their market value does not automatically imply that they are the highest in comparison to other types of stamps, which can vary widely based on rarity, demand, and historical significance.

In contrast, semi-postal stamps indeed have a fixed postage value, which is set at the time of their issuance, and they are regulated by the governing postal service that issues them. This ensures that their primary utility remains as postage while also fulfilling their intended purpose of fundraising. The characteristics of these stamps are specifically designed to highlight their dual mission of facilitating mail delivery and supporting charitable endeavors.