If you look at them they are all the same except for the name of the actual core, if you go to your Retroarch/Cores folder you can see them all and how they are named just copy that to the command line, so snes9x core would be -L "cores/snes9x_libretro.dll" for example.
I have not used Amazon pay to buy Bigbox, but I have used it for other things like Humble Bundles, and it worked just fine, you will just get a Amazon charge on your card rather than a Launchbox one.