A network-based transaction processing system allows users to flexibly and efficiently make payments using a combination of different payment instruments. The system includes a user interface through which a user can define one or more payment plans, each of which may specify a plurality of payment instruments and associated rules for dividing payment amounts among these payment instruments. These rules may be non-transaction-specific, such that a user can re-use a payment plan over time to complete multiple transactions. When placing an order or otherwise initiating a payment transaction, the user can select a payment plan for use in completing the transaction.