Why not telling me how much I owe

I have a question about the online site. On the first screen it says I owe 85.00 when I click make a payment it takes me to a screen that looks like the below screen. So I enter 85.00. Then on the next screen it says I owe 92.77. Why does it ask me to enter a amount when I don't know the final amount? At this point I am not sure if I made a payment or not because I was looking for a place to change the amount and clicked on something. Do you see the problem, It asks the customer how much they want to pay before telling them how much they owe.

Credit Card, Debit Card, or Checking Account

Enter an amount between $10 and $500.

Amount