Have you noticed that different banks and currency exchange shops have different exchange rates?
It is because exchange agencies have varying exchange rate fees. The "exchange rate fee" is calculated by adding a specific commission rate to the international mid-market rate (Google exchange rate). For example, if the mid-market rate (Google rate) is 1USD = 1,200KRW and the commission rate is 2%, the final rate will be 1USD = 1,176KRW.
With a 20% discount, the commission rate will change from 2% to 1.6%. Therefore, the rate after using the rate discount will be 1 USD = 1180.8 KRW, which is more beneficial than the original exchange rate(1,176 KRW).
Please note that the commission rate set by every money exchange stores and banks vary. WOW Exchange charges minimal commission rate and offers even further discount if you sign up!