Solutions for your two points are
1)Create 7 fields in html from. After press submit button, this will connect to your server, from your server it’s connect to third party with user name & password (user name, password setting into your server via script) so no need to type password it’s present into script.
2)Now , we will receive XML data (or xml file ) from third part correct. This data or file will parse (get XML data & put into HTML) using (SAX or DOM) on your server. Everything is present on your server so it can submit to shopping cart.
I will do bid correctly, once your requirement is clear or above my analysis is correct .I can do above things in JAVA, J2EE, XML.