Heya all.
I hope this is the right part of the forum (if not, feel free to move this post), but it looks like I've found a workaround for the Omni Plant bug (where you can't order Omni Plant seeds from the mailbox.) I found this originally on the official forum, but now I can't find the link.
Anyway, what you need to do, is to tell your sim to order the omni plant seeds, but then cancel the command before they get to the mailbox. The animation for them ordering the seeds will play out (but they won't be at the mailbox). I had my Omni plant seeds appear in the mailbox the next day.
Hope this helps people.