Along the lines of a radio button for "Full Amount" vs "Other Amount", have the Full Amount include any past due amounts.
It is confusing, email says I owe a certain amount, but clicking on "Pay Invoice" shows a lesser amount, namely the current month only!
That is, if one was overdue on paying the previous month, it is left out of the amount autofilled in the box at the top. But I expect it to show me the total I owe.
And there is no indication that the box is editable, just look at other entries asking for that feature. People don't realize it can be changed.
Perhaps a label to the right or below the box that says "Edit to set a different amount"
Or with radio buttons the Other Amount could blank out the box to make it obvious you can type there.
But even if I know that I can edit the box, I have to go back to the email to see the actual amount I have due. It would be handy to have a mini list of previous balance, current month and a total.
I hope someone is monitoring this page...