A computer implemented method of processing a payment transaction, comprising: receiving, at a network server, from an acquiring institution server, an authorization request message for requesting authorization of the payment transaction, the payment transaction having been initiated at a first merchant and the authorization request message comprising a transaction amount of the payment transaction; determining, at the network server, based on the authorization request message, that the payment transaction is eligible for a discount; calculating, at the network server, a discounted transaction amount for the payment transaction, based at least on the transaction amount and a first discount rule associated with the first merchant, wherein the first discount rule associated with the first merchant is one of a plurality of discount rules associated with the first merchant, and wherein at least one of the criteria defining the applicability of the first discount rule is that at least one of the plurality of data fields in the authorization request message comprises data representing a primary account number that is within a predetermined range; transmitting, from the network server to an issuing institution server, an updated authorization request message for requesting authorization of the payment transaction for the discounted transaction amount; and upon receiving a successful authorization response message from the issuing institution server in response to the updated authorization request message transmitting, from the network server to the acquiring institution server, a further authorization response message for authorizing the payment transaction for the discounted transaction amount. receiving, at a network server, from an acquiring institution server, an authorization request message for requesting authorization of the payment transaction, the payment transaction having been initiated at a first merchant and the authorization request message comprising a transaction amount of the payment transaction; determining, at the network server, based on the authorization request message, that the payment transaction is eligible for a discount; calculating, at the network server, a discounted transaction amount for the payment transaction, based at least on the transaction amount and a first discount rule associated with the first merchant, wherein the first discount rule associated with the first merchant is one of a plurality of discount rules associated with the first merchant, and wherein at least one of the criteria defining the applicability of the first discount rule is that at least one of the plurality of data fields in the authorization request message comprises data representing a primary account number that is within a predetermined range; transmitting, from the network server to an issuing institution server, an updated authorization request message for requesting authorization of the payment transaction for the discounted transaction amount; and upon receiving a successful authorization response message from the issuing institution server in response to the updated authorization request message transmitting, from the network server to the acquiring institution server, a further authorization response message for authorizing the payment transaction for the discounted transaction amount.