Sir/Madam,
This email is in reference to the order ID ______ (mention number). I would like to inform you that I have made the payment of ________ (mention amount) for the same order ID mentioned above but unknowingly, I have over paid _______ (amount).
So, I request you to kindly refund the additional amount I have paid as soon as possible. I apologise for my mistake and it would be highly appreciated if you could accept my apology and do the needful.
Thank you for your kind understanding and cooperation.
Regards,
___________ (Name),
___________ (Contact details)