The mailbox? On your property? that you paid for an installed (or bought off the previous owner), is government/usps property and they'll steal a parcel that someone else has delivered to it?
No, it goes further than that, all the way back to 1934. USPS is the only authorized service to use a mailbox. Here is an altogether far too detailed study of the law: https://www.gao.gov/assets/ggd-97-85.pdf
You need separate bins/boxes/whatever for other services to use.
