Would be "send additional books to this requestor" and then you could just go click click on the extra books you want to mail instead of having to do the "mail to a friend" thing and have a list of extra books to figure out which goes with which later when you're doing deals on the book bazaar.  I've been doing 3 for 1 lately getting ready to move and it's been a bit confusing sometimes when packing everything up! :)

Just an idea.

Oh and a benefit for that... it would then calculate the combined weight/media mail rate for me like it would if the requestor had asked for multiple books from me the normal way (all with credits).  My postal scale was ruined by a "helpful" toddler a few months ago so I don't have that option these days... I've been dealing w/ DH over the phone calculating media mail rate online while he's at the post office using their scale after hours.

I have been removing my "extra books" just by checking and deleting them from my shelf.  Am I supposed to use mail to a friend instead? 

Becky: No you're fine.  At least, that's how I do it as well. I think the OP is looking for an easy way to have PBS postage auto-calculate the weight--she's using Mail a Friend to do that right now.

Well, you can always manually adjust the weight on the book for Mail to a Friend so that it will recalculate the postage needed for multiple books.  Even without a scale, you can estimate what the combined weight is and change that information and have it recalculate.  Then you can just print one wrapper, wrap all the books together and you're good.