The special cargo is something precious. This usually means jewels, which are small and maybe easily smuggled. Probably not too valuable, or it might be sold by the carrier. Alternatively, it could be a message or token of alliance being carried. It might not need to be secret, but if it was, it might still be delivered.
With something secret, it might be hidden well enough to not be picked up by the pirates collecting the PCs. Or a token might not be recognized as important and left the PC.
Should the PC deliver this item, it could be a nice small loot drop for a reward.
Thanks for the tip. Though hmm I was thinking of making it some sort cargo/supplies for Harrigan. What better way to smuggle in aid/spies than not to tell the smuggler, only for them to be gangpressed by the guy who needs it.
Hmm I'll think on it.