I might get blown up by hardcore photogs, but...
Since your shots sound like they will be still subjects, you can probably save some money if you go the extension tube route for <$100. They are much cheaper and I'm assuming you'll be using a tripod so you won't need image stabilization, nor fast focusing. I think the main noticeable difference with the tubes is clarity fades as you get away from center. You might also benefit from one of those $30-40 macro lights which attach to the end of the lens.
I've never used extension tubes, so i'm not sure how they would affect the minimum focus distance. The Macro lenses should allow you to work quite closely to the subject. But yes, focus speed, and IS aren't important in this case.
If you really want great product shots, get an off-camera flash. If you do manual flash (not hard to do) pick up a cheap Yongnuo for around $60-100 that can act as a slave, and a set of RF flash triggers for $30. The $100 on the flash + triggers will give you far better product images than $100-200 extra going from T3i to T4i. Well maybe $130 if you add a cheap umbrella and stand, but still you'll get far more "pop" than you would be getting the pricier body.
Also keep in mind that i'd lean towards the 100mm EF Macro lens just because if Canon downscale EF-S (mounts of crop bodies only) i'm guessing the lens resale values may drop.