I was hoping that you would be able to order more seeds, but if they can't be ordered at the mailbox, then you've got the same Omni seed but that everyone else seems to have. I really don't know why you can't feed your Omni plants. Let's walk through this one more time. You have fully mature Omni plants. When you click on them and select "Feed" nothing happens. But you do get the option to Feed, right? Maybe it's because you have a lot of things in your inventory. For example, if you have 100 apples, the game is going to pull up each apple individually for you to choose one for the Omni plant. It can take a very long time to pull up the inventory list if you've got a lot of stuff. Maybe try cleaning your inventory out except for the things you want to feed the Omni plant. Or have a lot of patience and wait a long time to see if the box eventually appears after you click Feed.