Welcome to the forum sbarnes!
Please take a moment to review the two top links in my signature regarding the forum rules as well as the thread on capitalization & punctuation on the forum. You can modify your post by hitting the modify button and correcting it yourself.
Now to your question. Omni plants that people get in the mail is actually a broken glitch in the game and you can't actually do anything with them. The only way to get Omni seeds is to order them through the mailbox after you have completed the chain of opportunities. However, the omni seed ordering has been glitched in the past. The way to get it done is to queue it up for your sim then before they reach the mailbox, cancel the action. When you cancel the action, your sim will make moves as if they were actually using the mailbox. The next day the seeds should arrive.
As to the statement that you lost two money trees, how did you lose the money trees? Money trees go dormant in the fall/winter/spring if they are outside but I have never had one die. The frost doesn't kill the plants - it just puts them in a dormant state and as long as you are planting outside, your garden will not be harvestable. A workaround for this is to build a greenhouse structure which in Sims means a room with windows, a roof and no floor. Your Sim will get the unfinished moodlet while in there but your plants will be protected from the elements and will no longer be dormant.
The only way to plant in a greenhouse is to use the moveObjects on cheat or to knock out a wall and plant and then put the wall back up before the rain/snow/hail can get in.
