I like to be helpful, so here's the short version.
Ammo may NOT be mailed, i.e. shipped via USPS- period. There is no exception for FFL's as with handgun shipments.
Ammo can be shipped via UPS or FedEx using their ground service only. It must be packaged in a sturdy cardboard box labeled with "ORM-D Cartridges, Small Arms" labels or the new diamond-shaped "Limited Quantity" symbol; you can find online PDF files for both. The ORM-D label is much smaller, but the Limited Quantity symbol doesn't tell everyone what's in the package- your choice.
Both carriers have limitations on ammo drop-off and pickup; consult their policy guidelines for details. In my experience, generally the most significant limitation is that you can't ship ammo from a drop box or a franchised retail storefront location.