Premium Bandai is Bandai's Online only webshop. They only deliver to specific regions and sell to them even. Japan, Hong Kong, and I think Singapore are the only ones (don't quote me on that). Once a kit has finished it's Pre-orders, that's it. There's no more to be had unless they do another batch or they re-release it at some point in the future.
Many times P-Bandai kits are just re-colors, but other times they're outright nearly entirely new kits or kits with lots of new parts.
- CrashmanX Reddit