I'm thinking it'd have to work something like this:
Every order over, say, a 1 or 2 week period? Gets 1 or 2 of the cartridges, along with a note stating this is for the competition, and the forum address. (and entry
thread I guess)
This means everyone knows what it is for, and perhaps attracts a few more people to the forums.
Now, rather than holding off voting till after all cartos are delivered, it can be opened up immediately, which means people can vote as they get it.
The downside is that potentially fewer will be interested in competing if they're told, say "it tastes a lot like flavour XYZ" and they don't like XYZ, so it should be encouraged that noone describes the flavour until mailing outs have finished.
Anyhows, within, say, 1 week of the final mailout (to ensure all people get their packages, get to try it, etc) the submissions close.
Then the top 15 submissions are collected for a final poll to vote on teh best name, and at the same time, you can have a separate poll for if the flavour should be kept (The benefit of more flavours in the mystery mix is that there's a higher likelihood of finding a 'keeper' but will still work with 1 I guess)
Sound good?