I just learned that some people are exploiting the fact that the mailman is a villager in skyblock. They used the villager as a trading partner to get goods that are not in the initial package.
Please note that I consider that cheating. I urge everyone who got items through this trick to destroy those. If I catch anyone with goods that are not supposed to be there they will lose the skyblock w/o refunds.
I made the courier now a enderman to stop that.