I feel I must give some apologies to the USPS site. I went there and sent in an email asking if there was any way to get the postage that I had lost yesterday when the Click-N-Ship label didn't print. Very soon after I got an automated reply which told me how to file for a refund. I feel much better now. It takes 14 days to process (to make sure that label doesn't get used, I guess), but at least I'm not out the money.
I'm still going to only print postage from PayPal, though. Simpler and I can use Media Mail.