DanceSafe also ships worldwide. The OP specifically asks which is the "best and cheapest." As for "best," sulphuric acid is sulphuric acid, but as for "cheapest," Marquis reagent from DS is almost 1/2 the price of EZ. I think $40 for a bottle of Marquis reagent is outrageous. At any rate, I'd prefer to put my money in the pocket of an active non-profit harm reduction organization than in that of a commercial venture.
On the other hand, the OP also asks about differentiation between MDMA and MDA. I'm not a proponent of the Robadope reagent myself. So, then I guess one would need to compare prices on Simons' reagent in addtion to Marquis, and I'm feeling too lazy to do so. My feeling is that since the OP is asking about "the cheapest," they are not interested in buying a whole box full of reagents. But if they are, then EZ extreme is probably their best deal.
As a disclosure, I am partial to DS as I have worked with them in the past, but note that I also recommended Enlighten with which I have no relationship whatsoever. And this site is partial to EZ because of their mutually beneficial relationship.