It happened to me once, and the PO said they cannot refund the signature confirmation fee since I bought the postage through eBay, and I should ask for a refund from eBay/Paypal. I chose not to go through the nightmare of calling eBay customer service for $2, but next time I will.
I now write "SIGNATURE REQUIRED" in big red letters above the label. It has worked so far...
I now write "SIGNATURE REQUIRED" in big red letters above the label. It has worked so far...