Also, stamped mail (untracked, stamps can be bought with cash) can be dropped off anonymously at USPS blue postal boxes if it is 13 oz or under.

Orders over 13 oz could simply be split into multiple sub-13 oz shipments, although then that means a possible increased risk of detecting a package.

It depends heavily on the stealth used; a distributor could easily put a small quantity of drugs in a cheap object that weighs several pounds.