>So, somehow all the form's values are added to the string being posted. May be there is a different way. Say, instead of the action somehow send a hidden input? E.g. I can add some extra functionality to set some extra hidden input to be btnRemoveOverdue. My code just checks for the 'btnRemoveOverdue' to be inside the string.
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>I'll try this idea now instead of altering the action.

If this is a post and if you see that all the form values are added in the query string received at the server for the URL received, this means there is something that manipulates the action before posting. The default is to receive the URL at the server level for just the action URL. However, the server variables will have all the form fields in the hit.