A method of real-time three-party transaction processing, comprising, at an intermediation server: receiving invoice data defining a price payable to a vendor for supplying a product to a customer; prior to initiation of a payment to the vendor for supplying the product: extracting, from the invoice data, a vendor identifier corresponding to the vendor; selecting, based on the invoice data, a customer identification mechanism; determining, in association with the invoice data, a customer identifier corresponding to the customer according to the selected customer identification mechanism, wherein, in response to a determination that the invoice data does not contain the customer identifier, determining the customer identifier comprises: obtaining a unique invoice identifier, wherein obtaining the unique invoice identifier comprises generating the unique invoice identifier and transmitting the unique invoice identifier to a vendor computing device for capture by a customer computing device; and receiving an association request from the customer computing device containing the unique invoice identifier and the customer identifier; retrieving a payer identifier corresponding to a payer associated with the customer; retrieving, based on at least the payer identifier, a transaction policy; according to the retrieved transaction policy, allocating an eligible portion of the price to the payer identifier, and allocating a remaining portion of the price to the customer identifier; and generating and transmitting payment data for initiating a payment of the eligible portion from the payer to the vendor. receiving invoice data defining a price payable to a vendor for supplying a product to a customer; prior to initiation of a payment to the vendor for supplying the product: extracting, from the invoice data, a vendor identifier corresponding to the vendor; selecting, based on the invoice data, a customer identification mechanism; determining, in association with the invoice data, a customer identifier corresponding to the customer according to the selected customer identification mechanism, wherein, in response to a determination that the invoice data does not contain the customer identifier, determining the customer identifier comprises: obtaining a unique invoice identifier, wherein obtaining the unique invoice identifier comprises generating the unique invoice identifier and transmitting the unique invoice identifier to a vendor computing device for capture by a customer computing device; and receiving an association request from the customer computing device containing the unique invoice identifier and the customer identifier; retrieving a payer identifier corresponding to a payer associated with the customer; retrieving, based on at least the payer identifier, a transaction policy; according to the retrieved transaction policy, allocating an eligible portion of the price to the payer identifier, and allocating a remaining portion of the price to the customer identifier; and generating and transmitting payment data for initiating a payment of the eligible portion from the payer to the vendor. extracting, from the invoice data, a vendor identifier corresponding to the vendor; selecting, based on the invoice data, a customer identification mechanism; determining, in association with the invoice data, a customer identifier corresponding to the customer according to the selected customer identification mechanism, wherein, in response to a determination that the invoice data does not contain the customer identifier, determining the customer identifier comprises: obtaining a unique invoice identifier, wherein obtaining the unique invoice identifier comprises generating the unique invoice identifier and transmitting the unique invoice identifier to a vendor computing device for capture by a customer computing device; and receiving an association request from the customer computing device containing the unique invoice identifier and the customer identifier; obtaining a unique invoice identifier, wherein obtaining the unique invoice identifier comprises generating the unique invoice identifier and transmitting the unique invoice identifier to a vendor computing device for capture by a customer computing device; and receiving an association request from the customer computing device containing the unique invoice identifier and the customer identifier; retrieving a payer identifier corresponding to a payer associated with the customer; retrieving, based on at least the payer identifier, a transaction policy; according to the retrieved transaction policy, allocating an eligible portion of the price to the payer identifier, and allocating a remaining portion of the price to the customer identifier; and generating and transmitting payment data for initiating a payment of the eligible portion from the payer to the vendor.