My first promotion mission was like that, and it sucked a great deal.
Just to save you future heartache, you can recycle your available promotion missions by either waiting for the mission board to refresh, travelling to a different system/starport/planetary base, or using the tried and true method of switching between open and solo play. What you see on the board is random, and the promotion opportunity will not go away if you don't take the first mission you see.
The superpowers can be a bit masochistic when deciding on promotion tasks; sometimes they insist you murder or smuggle or take insanely tedious missions (like the one you accepted - quite possibly the worst!), and other times they offer you a nice trading milk run and ask you to pop down to the shops to fetch something for them.
My third promotion opportunity opened my eyes to this. Up until then I thought you were offered only one mission, but then my glorious promotion to midshipman went pretty much exactly like this:
"HEY YOU! We're a prestigious organization! To attain the rank of midshipman, you have to travel to this sector, murder these three people, steal the tea they were carrying, and bring it back to me! AND IT HAD BETTER BE STOLEN! None of this "buy tea from some other place without killing anyone" crap, I WILL KNOW THE DIFFERENCE. And you'd better hurry, i'm parched and I want my murder tea."
Or... OR, you could travel to lave, buy some tea there, and bring it back to me.
You face a difficult choice, commander"