credit card fees

Hi  I just wanted to know what process is in this case.  I have a client who has turned on online invoices, and getting paid sometimes with credit card and or bpay.  I have just looked at their accounts reports and noticed that the card fees and not the same as what they are charged each month.  about half of it to be accurate.  is there a reason for this, my understanding is any charge was passed on when this paymen method was used but is this not the case??