from PBS (since I didn't add the weight of the box or the two extra books I included) and not have to go to the post office even if the package is now close to 5 lbs, not 3 lbs?

You can add stamps to the package.  :)

As long as the pre-printed postage is still being used on the package then you can add stamps to it to increase the postage value and not have to take it to the PO.  The reason for the requirement for taking packages over 13 ounces, that only use stamps as postage, to the PO is because of security.  They want a record of who shipped any package over that weight.  Pre-printed postage meets that requirement because there's a "paper trail" back to the person who purchased the postage.  You can't trace stamps back though, so that's why you have to take those to the PO and show ID.

Thank you both so much for your reply.  I love this site!