Security for web site accessing i via IWS

Seeking advice on security to and from a web site and the IBM i database...
I'm developing the back-end web services to support a redesign by a business partner of a website used for Bill Presentation and Payment. Customers' account details, including payment account info, are to be inputs to the web service, and obviously need to be secured as they traverse the communications links. I need to know that what I'm doing is sufficient -- at least that I've made what is a reasonable effort to protect customer data.

My web services use the IBM Integrated Web Services (IWS), calling i 7.1 RPG programs to retrieve and update our i database, and staging committed payment data for ACH transactions. PTF's for HTTP and IWS are current.

Here are the elements of my design:
1) The web site will be accessible to the user via SSL. This will be the responsibility of the web site developer.
2) All traffic to and from the IWS operations will use SSL, with a self-signed certificate provided by our SysAdmin to the web site developer.
3) The web site developer is responsible for user authentication prior to calling the web service operations.
4) When a user is authenticated, the web site script makes a call using SSL to a web service operation to retrieve the details of the customer account.
5) My RPG program that retrieves the account details generates a 10-digit "SessionID" which is returned with the account data to the web site.
6) Calls to subsequent operations (PostSettingschange and PostPayment) by the web site must include that SessionID, which I use to log the session activity to a log file, and to track what data has been sent to the web site.

I'm sure that there are no absolute guarantees of security, and that there may be a myriad of expensive add-ons to make the process incrementally "more secure", but have I missed something that would make my plan irresponsibly inadequate? Are there things about SSL that I need to check to ensure that SSL flaws exposed last summer have been remedied? What are other shops doing to handle these concerns?

Many thanks.
Michael Koester
Programmer/Analyst
DataEast