Do I need a moisture barrier? Say I'm installing natural stone tile over a wood subfloor: Going from bottom to top: subfloor, moisture barrier, backer board (screwed through moisture barrier to subfloor), mortar, tile? Sound right?

Unfortunately there isn't a hard rule for all. With this many different materials in play, backer boards can be very different, same with mortars and the tiles themselves. This is a question that can only be answered by the instructions on the specific materials you would consider using.

Is there no Free Shipping on ceramic products?

Usually our ceramic products are unable to be part of our Free Shipping program because of the weight of the material.

Can I install over ceramic tile?

That's a pretty broad question, but I'll try my best. Basically the ceramic would become your subfloor. Considering the flooring we sell, most of it flat and solid, even the vinyl planks (if they are a bit bendy), any subflooring would need to be clean, flat and level. Level usually means no more than 1/8" of change over any 10 foot span of flooring, though your specific floor will have specific numbers. Flat refers to bumpiness or texturing on the surface. Again, your specific flooring will have its own guidelines, but as a general rule if you have more then 1/8" variance in the texturing, you would need to use something like leveling compound to flatten that out before installing over it. Those are pretty broad guidelines, and you'll need to consult your documentation for your own specifics. The exclusion, among our standard products, would be solid hardwood. That cannot be installed over ceramic tiles.