If you had to tell them what was in the box, you might consider that "extruded ingots (to be used in a manufacturing process)" which is (I believe) a technically accurate description of the material.
Certainly as accurate a description as anything one reads or hears on the news these days.
Mail restrictions on loaded ammo, powder and primers is driven by the regulation of these materials as hazardous materials for shipment. DOT regulates that (in torturously detailed manner), and basically the post office says "we're not going to deal with it at all in the mail. Use another carrier"
Jacketed or lead bullets are inert metal and have no hazardous properties for shipping purposes (providing you don't drop them on your foot) such as flammability, explosive, poisonous, reactive, etc.
Sturdy packaging, proper addressing and postage should do it.