Neither primers nor ammo can be sent by US Mail. Empty, unprimed brass and bullets not loaded into cartridges can be. Postal regs.
Ammo up to 66 lbs can be shipped by UPS or FedEx GROUND, and does not incur a HAZMAT fee. New sticker for those shipments takes the place of the old ORM-D label.
Primers and powder not loaded into cartridges ARE HAZMAT. They can be shipped by UPS Ground, but you must meet several requirements for HAZMAT shipping regs, and pay a fairly hefty HAZMAT fee to the shipper.
Side note for those asking why ammo is OK, primers are not- Primers- when loaded into a cartridge case, will not mass detonate. The mass of the ammo soaks up some of the energy. But a bucket of nothing but primers would be a different matter- one primer going bang can make the rest of the bucket go BANG!
And that has happened in ammo factories- with fatal results.