I accidentally sent my XXXX/Sychrony payment of XXXX to XXXX Card on XX/XX/XXXX. The amount was not due until XX/XX/XXXX. I did not discover my error until XX/XX/XXXX. At that time, I set up an ACH payment on XX/XX/XXXX for my additional purchases plus the last balance of XXXX. These additional payments were not due until XX/XX/XXXX XXXX/Sychrony calculated interest not on the 10 days that the XXXX was past due but on average daily balance daily balance of XXXX. How they calculate that number I can not find out. 
There is a disconnect between in the credit card industry and the consumer. How can I be charged interest on something that is not past due? 
I have since got them to waive the late fee but not the excessive interest charge.