When you install couriers stash integration and then move its esp to optionals, and install the WMX compatible one instead, you'll end up with two files:
Integration - Couriers Stash.bsa
Integration - Couriers Stash+WMX.esp
Am I wrong to assume that this will cause the .bsa to be ignored by the game engine? It needs to have the same name as some plugin for the game to load it. So the WMX compatible plugin has to be renamed to have the same name as the original: Integration - Couriers Stash.esp
In mod organizer you need to load the original mod and the compatible esp as two separate mods, move the original esp to the optionals tab, then rename the compatible esp in the second mod to have the same name as the original.