Storing Credentials for a Third Party
I've written extensively about how to solve this problem, over on the IT Security Stack Exchange site.  Let me point you to my answers over there:


What is the best way to securely keep clear passwords?
How can I create a service that automatically logs onto a third-party service without storing credentials in plain text?
How should you store a password that needs to be retrieved as plaintext?


Basically, I'm suggesting you set up a separate machine that stores people's credentials for the third-party website as well as a hash of their local password, authenticates the user, and then connects directly to the third-party website.  That separate machine should be secured and locked down as carefully as possible, to reduce the likelihood that it is compromised.  But read those links for all sorts of extensive details and suggestions.