We do open and scan some percentage of regular mail at the post office. It's difficult to find exact sources because the USPS only seems to easily admit to doing it for postage reasons, but it's fairly well known that they search for drugs, bombs, etc. Mail is subject to X-ray scanning and being opened under suspicion of a variety of things happening. When they can't open it themselves, they're also allowed to request permission from the recipient (you can refuse, but then they can go to a judge).