This can be solved by adding a new number to item that will tell you what is the difference between the galactic shop:
Here it is +13 keys in red, because it is 13 keys more than in galactic shop and red color is usually seen as a loss. Gain could be green or blue.
Then maybe if shop is out of stock of their items then it uses the galactic store items? This way you can’t have something out of stock.