All Airlines charge fees for Infant ticket on Passport flights...only on domestic can airlines waive this. If you did check their web site you during your booking their is column with the question if you are travelling with an infant. If you answer yes, you will get all the details and fare for the routine.
Please do demand for your receipt as that is wrong, may be they were in a hurry at the point. Please there is no excuse not to give the receipt as that is evidence of exchange of money.
Why not contact them on
[email protected] or 012776619 to complain about this.
The best guy.