I have tried the "cancel action before Sim reaches mailbox" trick to order more Omni Plant Seeds. On my computer, it didn't help. However, after getting her initial six Omni Plant Seeds for finishing the opportunities that lead up to it, my Sim could plant, water, fertilize her plants. Once they grew up, the interaction to feed them appears, as does check on diet. However, as I have seen noted in an old post around here before posting this, she cannot FEED the plants anything. No fish, seed, fruit, books, or anything else. She cancels the interaction before even getting to the plant.
For me this has been going on since about the time World Adventures came out, though it could have been when Ambitions came out. I KNOW it was before Late Night, anyway. I've tried to explain the issue to support, and they just always tell me this is the first they have ever heard of it. (I know I've told them at least three times, so yeah, I really believe that. LOL)