In theory, if you give a bank an instruction, you are the principal and they are the agent, and that means they have to follow your instructions pertaining to y

In theory, if you give a bank an instruction, you are the principal and they are the agent, and that means they have to follow your instructions pertaining to your account. If it's a written instruction, they do need to authenticate it, but they could call you to do so. Or they might charge a big fee for it. Or they might drag their heels by claiming you have to perform the transaction online because of some policy they have that you don't care about. But they definitely can't ignore a letter you send them.