The postman was applying an ancient principle once elucidated by Chester Burnett as

See here, boy, I'd rather go to your funeral than have you come to mine!

Cocaine was used in an ointment for horses affected by warble flies. The ointment contained citronella oil to keep the flies away, and the cocaine was to soothe existing bites, and stop the horse bucking when bitten. You can still get ointments of this type, but these days they contain amethocaine, not cocaine.

A mojo is a Gris-Gris charm, usually contained in a little bag, and usually intended to give the owner power over women.

Deep River Blues - try a PM to Mrs Duck !