Remove paste restrictions on the account number
I have my routing and account number in a Google Doc for a situation like this when I pay from my account. There's no option to paste the account number in the second field. I keep those numbers so I can easily copy/paste them without a need to type them manually which causes mistakes in the first place. If this is a way to reduce the number of mistakes you accidentally made it more error prone by forcing me to put the number manually.