So, I accidentally bought a cheap captain commission. I've been trying to get rid of it: putting it on the table and then cancelling it doesn't work--the next session, it's back. So I decided to actually go on the voyage and try to complete it.
Well, it being a Merchant Alliance voyage, it gave me like 12 animal crates, and there's no way I was going to run around playing Pokemon. So I did everything else and then left the game.
The voyage STILL came back.
This tells me that so long as you don't actually COMPLETE a commission--do 3 out of 4 of the quests it gives you, and then ignore the rest--you can endlessly use the same commission over and over. This is doubly true if you only care about one mission on the commission, e.g. the sunken ship part of a Merchant voyage.
This might be related to all the server bugs I and others have been experiencing with ship cosmetics reverting, captain logs not saving between sessions, etc. etc.: the merchant commission itself might just be endlessly reverting when the intended effect is to have it clear.
Regardless, whatever hotfix they come out with for the other issues might want to address this one.
