I got a PBS book in the mail today. It had stamps that were marked through with a sharpie and someone had written "22 cents due" on the front under the stamps with the same color sharpie. That was all.

I've had a postage due book before and they sent a little card to tell me how much was due and I had to pay it before they gave me the package. 

Is this package postage due? I'm confused.

yes, your carrier paid it out of his/her pocket so you could have the package with no delays. All you have to do is put the correct change in your mailbox, either in a well marked envelope or taped to a piece of paper. 

Thanks for letting me know! I'll put it out right away.