Well, this simple example works for me:
Code:
funPrintByExtension("foo.txt")
  
Function funPrintByExtension($sFileName)
	Dim $oShell
	$oShell=CreateObject("Shell.Application")
	$oShell.ShellExecute($sFileName,"","","print",1)
	If @ERROR "Error in open: "+@ERROR+": "+@SERROR+@CRLF Exit @ERROR EndIf
EndFunction


This prints the file "foo.txt" to my default printer via notepad.

Change the "1" at the end of the line to "0" to hide the notepad window.