How must lithium batteries be mailed?

Lithium batteries must be mailed using retail ground services due to safety regulations surrounding their transport. These batteries can pose a fire hazard if damaged or incorrectly handled, which is why many carriers have specific guidelines for their shipment. Ground transport allows for more stringent control and safety measures compared to air transport, where the risks associated with fire need to be managed more carefully due to altitude and pressure variations.

While express service or air transport options might be available, they may carry higher risks due to the nature of lithium batteries. Standard delivery options may not provide the level of safety precautions that ground services do, making them less suitable for shipping such items. Thus, retail ground is the correct answer as it aligns with regulatory practices and safety protocols for the transportation of lithium batteries.