Case 2 There is not really any agreement or that recycling old helmets allows the store to recover some of its costs. It's about a differentiated pricing method that takes into account the elasticity of demand of different consumers. That is to say, given a certain percentage of price change, the buyer demand quantity change is called the elasticity of demand, and the change is called less elastic. Higher prices are set for less elastic buyers and lower prices are charged to customers with more elastic demand. Although the merchant does not gain anything by exchanging the old safety helmet, the merchant can use whether or not the customer takes the old helmet to differentiate the customer's elasticity of demand. If the consumer does not take the old helmet, it means that he does not have a helmet, but the relevant laws provide that he must wear a helmet (such as motorcycle driving), so no matter how high or low the price is, he must buy it, and this kind of customer's demand curve is steeper and less elastic. On the contrary, take the old helmet to offset the price of 20% off the customer shows that he already has a helmet, so his demand for new helmets is not so urgent, this kind of customer demand curve is flat, elasticity is bigger, through the "old hat for a new hat all 20% off" can be very good to stimulate them to go to a new cap of the demand.
Case 3 This is the result of the market economy. Star belongs to the scarce resources, the market supply is insufficient, can be said to have a monopoly in the market, however, the market demand for the star is very large, according to the law of supply and demand, the supply and price is the reverse change, so from the economic point of view, the singer has a very high income in line with the laws of the economy.