From Reason Magazine:

Though the USPS shipping ban in Congress is a wholly federal matter, state shipping bans stem largely from states’ authority to regulate alcohol under the 21st Amendment. While that amendment ushered in the end of federal alcohol Prohibition, it also, I explained in 2017, “basically transferred the language from federal prohibition and made it essentially state prohibition, so it gives the states this incredible power to do terrible things, and not surprisingly, the states have exercised that power with great vigor and glee.”