Oh, phooey. I just noticed that I bought Osmocote Flower & Vegetable Plant Food instead of Osmocote®Plus Outdoor & Indoor Plant Food.
The receipt I sent in with the rebate form just said "plant food," but it probably had a numerical code that would disqualify me. Oh, well. I needed some anyway, so I won't complain if I don't get the rebate. And next time I'll make sure to buy the correct product.