I would see this as a bit of a conflict. You are basically expecting your customer to pay you extra, through a registrar, for work that they would already be expecting. Could be considered a hidden fee. In real estate, it is 100% commission driven. The parties expect to pay XX% - it does not matter how the sales people split it up.
Back when I consulted it was part of the customers expectations (and the contract) that I assist with registrar selection. I had several I had worked with and recommended. The client made the final call. It was a much biger benefit to me to provide this good customer service which resulted in more referral business from my clients. Also getting to know the registrars and auditors got me more business, since they would recognize companies that needed help, and point them to me for assistance. Referrals back and forth were worth much more than a few bucks here and there.
Consider this situation. Registrar A has an excellent reputation, very fair, provides value added auditing, no comission for the referral. Registrar B is so-so, complies with requirements, but is bad at scheduling, does the minimum, unexperienced auditors, etc. - and provides a $500 commission. Both are charging your client the same amount. Which are you going to recommend? Is it what is best for your client? Are you going to disclose the $500 to your client if you point them that way?