Do you charge a broker fee?

Lynn Riley recently asked this question on the Creative Freelancer Conference LinkedIn Group:
I'm curious if you all charge a broker fee to a client if you do the task of gathering bids from printers + mailhouses and serve as the client/printer liaison? If so, how are you handling the invoice?
A few group members have shared their experiences. Kathleen Hayes commented:
Usually, I procure the printing/fulfillment. I mark it up and pass it on to the client. The markup is included in my original estimate to the client. If the client wants to pay for printing/fulfillment directly, I charge an hourly rate for any "print management" I do on their behalf: from gathering competitive bids to liaising with the printer to facilitate the flow of the job. I might also do this for a fixed fee, it would depend on the client. 
Nicholas Nawroth commented:
If I am producing the design for the client that will be printed, I make sure to account for the time it will take me to work with the printer: gathering the quote, prepping the job, etc. It all included in my final project quote that I present to the client. I never mark up printing since the client can simply have me e-mail them the file and then go to the same printer and save whatever commission I would charge. However, my client always pays the printer directly, I don't mess with trying to pay for printing then have the client pay me, too much trouble! 
Rochelle Weiner commented:
I work pretty much like Nicholas – I include print estimating, liaison, press check etc in my original estimate at my hourly rate. (However, I job out the print estimating to another associate). If the client has a printer they like to use we include them in the estimating process. The client pays the printer directly – I don't want to get into that either. I've been working this way for years – it's a transparent process and my clients seem to appreciate it. 
What do you think? How do you handle this situation?