This song is Keith Richard's "ode to heroin."
"Little Susie" is a reference to the "brown-eyed susan" which is a metaphor for brown heroin, one of the more common forms of the opiate.
"Roses" is yet another flower which has been used as a metaphor or "street slang" for many years to allude to heroin.
"Dead Flowers" in general means "poppies which have been harvested and from which the opiate has been derived."
The power of heroin to kill pain is legendary -including the emotional pain from a broken relationship.
In the 1960's and 70's one way to deliver heroin was to place small amounts in a letter and use the US Postal System. Back then, before the days of technology to detect narcotics in the mail, and in the days when the American political system actually observed people's rights to 4th amendment privacy, heroin could be sent through the mail virtually risk-free to either the sender or the receiver.