"By" isn't needed here, but it is true that "by" is often used with "due" -- especially when saying that something is due at or before the end of some time period, or at or before some action. You might say the money is due BY the end of the week. Or BY the moment of departure. Or BY the time that you leave. It would be incorrect to say "the balance of payment is due the time of departure." You would need "by" in a case like that.