A supplemental financial transaction processing system operates with a primary system to provide financial transaction processing services to a client system which has established a secure session with the primary system. A secure web services system receives a transaction request identifying the primary system and: i) assigns a unique redirect URL to the transaction request, and ii) returns the unique redirect URL to the primary system. The primary system provides the unique redirect URL to the client system. The supplemental transaction server: i) provides a web document object to the client system; ii) receives a post of the financial transaction from the client system; and iii) performs at least one of: i) writing the financial transaction to a transaction database; or ii) forwarding the financial transaction to a processing system distinct from the supplemental transaction processing system.