Answer to rhetorical question 1: if a sending post office charges inadequate postage, the delivering post office can add postage due.

If you use a commercial account (such as PayPal, click-n-ship, etc.) a new policy being implemented will deliver that package without postage due and will charge the underpayment back to your sender account. People will get upset, I'm sure, but this is far better than having a disgruntled customer complain that you didn't ship it properly. This will apparently only apply to package rate - it does not cover those who send using the letter rate. I recently received a notice from one of the mailing services but can't put my hands on a link.