Wrong Total Billing

How did you pay? If by bank, it takes about five days to register and you might have had to pay late fees.

 Did you download your invoice to see what you were charged for exactly?

You also mentioned that "pay it last day," it means your bill was generated before they received the payment.  Check your due amount 5 days or more. It should be showing adjusted amount 

Have you checked your most recent bank statement? If it is not showing up, the payment may not be successful and you should follow up with the bank to inquire about the issue.

I would also suggest you to check your Self-Serve account for the outstanding balance to see whether or not it is different from the one on the bill. A lower amount would imply that the payment actually went through and it was updated after the bill was issued.