(20)The caps in this Regulation are based on the so-called ‘Merchant Indifference Test’ developed in economic literature, which identifies the fee level a merchant would be willing to pay if the merchant were to compare the cost of the customer's use of a payment card with those of non-card (cash) payments (taking into account the fee for service paid to acquiring banks, i.e. the merchant service charge and the interchange fee). It thereby stimulates the use of efficient payment instruments through the promotion of those cards that provide higher transactional benefits, while at the same time preventing disproportionate merchant fees, which would impose hidden costs on other consumers. Excessive merchant fees might otherwise arise due to the collective interchange fee arrangements, as merchants are reluctant to turn down costly payment instruments for fear of losing business. Experience has shown that those levels are proportionate, as they do not call into question the operation of international card schemes and payment service providers. They also provide benefits for merchants and consumers and provide legal certainty.