First, wording is essential here. The cup/item can be used by *a* true king, not *the* true king. This will enable any orc king who happens upon the item to use it, thus make it more of a threat. If it is the king's uncle who hires the thief, he will be the rightful heir when the child-king dies, thus also able to use the cup/item when that happens.

If we're going to continue to use the cup, then there must be some sort of range on it. Maybe it has to stay within the primary dwelling of the king in question? In his bedchamber? Otherwise a wearable item or trinket makes more sense.

I think that the original thief should, like jtuogas says, pass on the item to someone else for transport on the caravan. Then the original thief should be caught and imprisoned, imo. This will give the PCs someone to interrogate and possibly glean some information from. In 4e it would be a skill challenge; success would give them a certain bit of information, while failure may lead them to a difficult combat encounter before learning the same information. The thief's opening monologue could be from a jail cell, confident that his guild brethren will somehow set him free.