Let's say you want a store to offer a 50% Discount even though the store is stocked with common items you can buy anywhere else without every store also offering said discount. This is do-able.
-In your Database, duplicate the items relevant and alter their Price so that you now have "Full Price Versions" and "Discount Price Versions."
EVENT (the shop clerk offering the discount)
-Make Variables for each Item.
-SET the value of each VARIABLE to the ITEM and Number Held.
-Remove "Full Price Versions" of each item by the respective Variable
-Add "Discount Price Versions" ...
-Call Shop, stocked with "Discount Price Versions"
-SET the values of the VARIABLES again.
-Now Remove "Discount Price Versions" ...
-Add "Full Price Versions" ...