At the top of my head:
1) First come first serve among all the Top 50 in Ranked Battle Rankings of the server. Fastest fingers first. (You probably already know this).
2) If the person is already successfully plundered, he will not appear on the list again. So technically he can run around Konoha with that Golden SS above his head without fear, since he no longer have to rush to reach the final destination (except against that 30mins convoy clock). So seeing him run past you, you hurry into your group base but not see him, cause he's either already been successfully plundered, or something click him in the list in that 8 seconds that you rank to that plunder NPC.
3) You can be at most 15 levels more than the person who does the convoy. (i.e. if you're level 85. You will only see level 70 and above convoys.) So, level 67 and lower players in older servers, feel free to convoy SS everyday, since the Top 50 is normally all at least 83.

That's all I think, if I left out stuff, others feel free to correct me or chip in! :)

* that 15 level thing is annoying, but thank you so much, didn't know that one

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