When doing a custom withdrawal, make the elimination of a $100 bill automatically create five $20 bills. Also vice versa; when adding a $100 bill automatically reduce the number of $20 bills. Since you cannot exit with a total different from what you entered, this still allows all possibilities to be done essentially replacing six clicks by one.
Even better, start the custom process with the minimum number of bills rather than all $20.
Currently, when I want $400 as 3 $100 and 5 $20, I have to click fifteen times to eliminate the unwanted $20 and three more times to get the $100 for a total of 18 clicks. With this suggestion, one would do.