I like to help out PBS by filling out the info about amount of postage, postmark etc ,

Several times when I have received 2 or 3 books from a sender (a bulk order in one envelope) I've seen no way to do anything but acknowledge each as a separate book received.

Since the actual postage rate on the envelope is based on multiple books,  I can't use THAT amount for each of 2 or 3 books.

In the end I simply don't  fill out that portion.

Is there any way to do a "fix it" so that a bulk order can be properly acknowledged?

No, but you could note in the general comments box that the book was part of a bulk shipment.

I do the same as the above, for each one, I fill out the total postage, and then in the remarks for TPTB (actually I just started doing this, but I will from now on) I remark that it was a multiple book shipment.

I do what Jane and Mimi do, fill it out & note that it was a bulk shipment.