Web Page Listener For POST Message Problem

I created listener for POST message from client as a simple web page and process that message in Page_Load method. If I use url for posting message to my listener with explicit web page name http://myserver/formhandler/listener.aspx it works without error.

But my clients will need to post message using url without mentioning page name like this http://myserver/formhandler. In IIS for formhandler virtual directory I set default document as listener.aspx. And now my issue starts: server responds every time with error 405 Method not allowed. I set properly mapping for aspx extension in configuration to allow POST method.