Another risk: there are various small rules for individual countries on what you can't carry in and a person carrying somebody else's luggage may not know those

Another risk: there are various small rules for individual countries on what you can't carry in and a person carrying somebody else's luggage may not know those specific rules. (For example, flying into the Middle East with certain types of religious literature is a no-no). Huge risk for the guys carrying somebody else's personal items, even if they appear to be safe and innocent. International courier services handle a lot of risk, offer insurance, and have really improved the past few decades globally since every ships online these days.