Hmm I don't touch much of the pricing/logistics side of the business, so I don't really know why the prices move that much on Amazon. But I can generate a discount code for you guys to use if you buy stuff directly from the 2hr Aquarist website: 95GF77H for 15% off - this should work universally.
I've used aquasoil to cap raw soil for years. I find that this combi grows plants better than using pure aquasoil (no matter what producer). However, I found most folks not interested in the idea as it seemed messy, and there were issues with raw soil selection due to its non uniform nature, so I stopped writing about it.