And then you add a little more when printing postage?

They order one book from you and then PM you with the freebie they want. You remove the freebie from your shelf.  THen you wrap them together and adjust the postage if using printed postage. It really helps to have a little scale because the weight that PBS lists isn't always right.  If you don't have a scale you might want to see if the weight is listed on Amazon or somewhere just to check it out.  The PBS postal calculator will adjust the postage as you adjust the weight.

I usually write the name of the freebie book on the wrapper when I print it out. 

I don't know if others would want to do this because it involves trust.  I recently made a deal as the receiver.  I picked out the books I wanted and then told the sender to just give me a credit back.  She was so excited because she said that made it easier for her to do the postage and didn't have to worry about removing the books from her shelf.  She has over 500!  A lot more than me.

As soon as she marked the books mailed I received the credit.  It was a good deal for both of us.

I must admit the person was someone that I had seen in the forums and felt very comfortable doing it that way.  I was the one to suggest it to her.  I don't think I would have to a "stranger."

I'd trust Mary. :-))

I've done that before too, and it's certainly handy if your requestors have the credits on hand to order them all instead of just requesting the freebies.  If I advertised my specials more in the BB, I'd put that in my ads as an ordering option, because the BB advertised specials are where you tend to have the problems with duplicate requests or people ordering books that have been requested as freebies.  It's nice to be able to print bulk labels, especially when you use PBS postage, and not having to run & pull freebie requests 20 times in one day is great too:P  Of course, it all depends on whether or not your requestors are willing to trust you to refund the additional credits, and whether they have them on hand to order everything.  If you're running a 5 for 1 deal or something, it can be hard for anyone who doesn't have surplus credits.