Hey there, Dane from BookRazor here.
As Evie said, your mileage may vary when using our service. I'm in constant communication with all of our clients, and there are so many variables to consider with regards to a response rate, such as the similar books submitted, the headline of the email sent, to even the book genre itself.
With that being said, I do offer a 100% guarantee on all our services, so even if you try it out and are dissatisfied with the results (whatever the reason may be), I'll gladly refund your payment.
Lastly, we used to send out the emails on behalf of the author, but we've found it to be more well-received when the author personally does this. You can develop a relationship with these contacts, which may come in handy if you write a sequel in the same vertical or have an ARC, and your list, while maybe a one-time purchase from us, can be used over and over again.