If the seller wants paypal protection, they need to use a trackable shipping option. (some will use a cheaper non-tracking option, if they want to take a risk or are new enough to ebay to not yet be scammed by a buyer)
Which is unfortunate that honest buyers have to pay for the actions of a dishonest few.

Just note that USPS is the postal service (like Australia Post here, and arrives here through Australia Post), while UPS is a courier service (which is usually more expensive, but requires you to be home to personally sign for the items, as UPS deliver it themselves here).