Many schedulers have a mail merge program where they can send out a blast email and a form email and use terms in the mail like:
<Firstname>, I would really take it as a personal favor if you could do this shop for me. You have done this same shop for <Customer> in the past with excellent reporting.
It may appear personalized to you, but it's still a form letter. I never reply to a form letter, unless I'm attempting to take the assignment.
Save the schedulers inbox from being cluttered.
Unless they mention something in the email that is 100% personal to you, assume it's a form letter that has some personalization data fields added to it.
Your not doing anyone a favor by sending a rejection reply unless you really know the scheduler and they are asking you to shop immediately and offering a big bonus.