I want to grant Bill Pay access to a particular account but the permission is not showing up in the list for that account. Why is this?

The Bill Pay permission will only display for accounts that you have already setup as bill pay funding accounts. If you want to grant Bill Pay permissions to an account that is not currently a funding account, you must first go to the Bill Pay page and add this account as a funding account and then attempt to add a subuser again. The Bill Pay permission should then display for this account.