When I receive a large multiple order, I ask the requester to select another book from my bookshelf as a thank you gift for the multi order. I certainly don't expect a credit for the freebie but I wonder if there's anyway to note the additional book during the label/postage printing process.

I know I can't use 'mail to a friend' off my own bookshelf. I'm only trying to purchase the correct PBS postage. In most cases I need to add onto the PBS postage at the post office to cover the additional book.

I'm not sure why you can not use the "mail to a friend" feature. 

I use it as follows: 

I accept the multiple order. Select the "hand write address" option. This will give you the recipient's address. Keep it handy. 

I go to the PBS page of the freebie and select "mail to a friend." I copy-and-paste the recipient's address. It asks you to confirm. 

On the main PBS page it would ask me to "reprint" the label, now with the freebie + ordered books. If you have a scale, it would be a good idea to check the combined weights. 

I don't use PBS postage, but I use this function to keep track of the postable freebies on my shelf.

Hope this helps.

Thanks.