There are many ways to transfer data between forms and server.  Using ASP.NET most can only be done on the same server or application.

Fortunately dot.net 2.0 gives us the ability to use the postbackurl tag (in an asp:button) which can be redirected to another server eg: for third party payment pages.

When the .aspx page renders, it changes the code to webform_dopostbackwithoptions 
Check your rendered page (view source from browser) to see exactly what i mean.  Also you will see a 

type="text/javascript" script src="/WebResource.axd?...

If you pages isn’t posting back to another URL and just appears to be posting to itself (you may see a quick page error in the bottom corner) check that your asp.net file mappings are enabled on the server.

You WILL need to have .aspx and .axd file extensions mapped to the aspnet_isapi.dll
and make sure that the ‘Verify that file’ exists is UNCHECKED for the .axd

http://support.microsoft.com/kb/815175

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