"I'm calling about those Dodos you were offering during the 1612 Summer Sales Event. You had them marked down to $2.99. Since you're not selling them anymore (I heard they were extinct), and I wanted them, you have to give me $100 off that new television."
A Dodo Discount occurs whenever a customer sees an item being advertised at sale price, it becomes discontinued when they're trying to order it, and they ask to transfer the missed savings onto an unrelated product due to their trouble with getting it when it was on sale. Ignoring things like supply and demand, economics, or "first come first serve", they believe they are entitled to deep pocket discounts on other items whenever something they want becomes discontinued.