1. A multi-account payment consolidation system, comprising: a central computer system operatively connected through a network connection to a payment terminal at a point-of-sale, the central computer system including at least: a processor; and memory comprising program instructions, wherein the program instructions are executable by the processor to: register a plurality of external financial accounts and associate them with a user; store at least one user-defined customized rule-set to process a payment associated with the user; receive a request to initiate payment of a transaction from the payment terminal; and automatically process the payment in accordance with the customized rule-set by accessing the external financial accounts, wherein the customized rule-set directs at least a rule that the payment of the transaction be apportioned between more than one of the external financial accounts.