I had a little think earlier on and I came up with an idea. There should be 2 different classes of smuggling vehicles. Class 1; Mule; Sherbet; Yankee.
Class 2; Vapid ST Van MK2; Ford transit.
Class One: Can smuggle all items available from the smuggler(Ammunition; TNT C4; Bomb; Explosives)
Class Two: Can smuggle only the first 2 items available from the smuggler(Ammunition; TNT)
I think this would be a lot better, affordable, and less suspicious than being a main target in a mule.