I recently sent a package using PBS postage. Apparently I put the wrong weight in and the postage came up short. It was returned to me with a note saying that it needed 17 cents additional postage.

My question is: Do I have to print a new label with new postage or can I just put additional stamps onto the label and resend?

Hmm. I'm thinking you should be able to just add the postage difference.  You might want to call your local PO and ask just to be sure. 

Actually with the printed postage you should bring into the PO.  They are picky about that printed postage date.  If you stick a stamp on it and send it off it may come back as being mailed too late.  If you take it to the PO adn it's still good then they can add a fresh post office generated stamp for the difference and update the date.

My postage date is always the date that I mail it. I usually do not print until I am ready to mail. If it is under 13 oz,  I typically do not go into the PO.

Just stick a 17 cent stamp on it and give it back to your carrier.  It should be fine.

faq.usps.com/eCustomer/iq/usps/request.do

Note: A returned shortpaid mailpiece can have the necessary additional postage affixed (leaving the "postage due" message intact) to the original piece and does not have to be placed in a new envelope or wrapper.