I made a deal with another member. I would like to use PBS printed postage for all the books to send to her. How can I do that since it only shows the books she ordered?

You need to get the weight of all the books together and then change the weight that the site gives to the weight that you come up with.  You can go to the PO and use their lobby scale (take along the box you will be using for a more accurate weight).  Or you may have a postage or food scale around the house you can use (again, figure in the weight of wrapping).  Or, finally, you can check all the books by ISBN # on Amazon and get the weights from their description sections.  Add them all up, figuring every 16 oz. is a lb. and be sure to allow a little more weight for the wrapping.  It can be a pain, but it generally works.

Use mail to a friend for each book in the deal and the books will be added together. The weight shown will be closer (probably) to the actual weight. It's still a good idea to use a scale if you're printing your own postage. I usually get the PDF label for the main book to get their address and on mail to a friend type that address in. The PBS system will recognize and combine the order. Once they're all together you can generate the real label with postage.

Get the first label with no services. Use that address to use mail to a friend. Only then, when the books combine, print the postage. 

Thanks everyone! I did as Emily suggested. I got the first label with no services. I used that address for each of the other 5 books as mail to a friend. They all came up with the new weight and I used PBS printable postage,