This Seeking Tech guide will explain how you can unlock the Shrewd Shopper trophy and achievement in Metaphor ReFantazio.
For the first step, make sure that you are on Idelsday, which is a day that ends with a 0 or 5. When you bring up the map, you should see that some shops are in red, which indicates that they are having a sale.
When you buy a discounted product from one of these stores, the Shrewd Shopper trophy should unlock.