A supplemental financial transaction processing system operates in conjunction with one or more primary financial services systems for purposes of providing supplemental financial transaction processing services to users of the primary financial services system. The supplemental financial transaction processing system comprises a secure web services system and a supplemental transaction server. The secure web services system comprises at least one financial transaction method available to the primary financial services system. Upon receipt of a method call for a financial transaction, the supplemental financial transaction processing system: i) associates a unique redirect URL to the method call; and ii) returns the unique redirect URL to the primary financial services system. The primary financial services system may then pass the unique redirect URL through a secure connection with its client. The supplemental transaction server, upon a client requesting an HTTPS connection to the unique redirect URL, verifies the validity of the unique redirect URL. If valid, a web document object is provided for rendering by the client. The web document object comprises: i) the financial institution defined transaction values (at least one); ii) user interface control script for rendering of controls for obtaining user entry of at least one client defined transaction value; and iii) transaction control script. The transaction control script comprises instructions for i) creating a financial transaction; ii) validating the financial transaction; and iii) posting the validated transaction back to the supplemental financial transaction server.