1. A method comprising: receiving, by a merchant server, an indication that a customer accepts an offer for a subsidy from a second merchant to be applied to a purchase of at least one item from a first merchant, the offer requiring the customer to agree to participate in an additional transaction with the second merchant; in response to receiving the indication of the acceptance of the offer, reducing by the merchant server a price of the at least one item for the customer from a first price to a second price, the second price being less than the first price; after receiving the indication of the acceptance of the offer, selling the at least one item to the customer for the first price; after selling the at least one item to the customer, determining by the merchant server that the customer did not participate in an additional transaction with the second merchant as the customer agreed to; and in response to determining that the customer did not participate in an additional transaction with the second merchant as the customer agreed to, assessing by the merchant server a penalty against the customer.