I had the same problem with ‘Purchase In Progress’ hanging and not doing anything, and although most posts are trying to solve this there is no good one for all explanation, so here is the fix.
At top of Steam / Settings / In Game - put a check mark in the box to enable Steam Overlay.
Also you have to enable the same setting on the actual game. go to your MS Flight Sim on the game list in steam, Right Click and select Properties, you will see there is also an enable steam overlay here that needs checking too, once this is done, load game and you should be able to purchase from the market place.