What does a semi-postal stamp pay for?

A semi-postal stamp primarily serves a dual function. It covers the cost of first-class single-piece postage, which is essential for sending standard letters and lightweight packages through the postal system. In addition to this basic postal service, the extra fee associated with a semi-postal stamp is designated for contributions to public interest causes, such as medical research or humanitarian efforts. This means that every purchase of a semi-postal stamp not only facilitates mailing but also supports important social initiatives, thus embodying both a practical and philanthropic purpose.

The option that describes only international postage fees does not align with the purpose of a semi-postal stamp, as these stamps are typically used for domestic mailing. Similarly, insurance and tracking fees are separate services that are not covered under the cost of a semi-postal stamp. While additional delivery services could involve various enhancements offered by postal services, they are not the primary focus of what a semi-postal stamp specifically pays for. Hence, the answer accurately captures both the postal and charitable elements inherent in using a semi-postal stamp.