According to the plea agreement, law enforcement officials became aware of Harris' criminal conduct in February 2015, when a package was intercepted at the Louisville hub of UPS. A search warrant executed on the package revealed a large quantity of suspected methamphetamine being shipped from California to Todd County. State and federal law enforcement officials conducted a controlled delivery of the package, that contained approximately 18 pounds of crystal methamphetamine, and observed Harris taking possession of the package and placing it inside a black van.
No. Your understanding is slightly flawed. UPS employees can look into any package you send to ascertain it's contents if they so please. If they find something illegal and decide to inform LE then LE is required to get a warrant to make it legal. The warrant is a formality at that point as UPS will stop the delivery process and wait for LE to show up with warrant to officially hand package over. Private companies that you ship packages with aren't constrained by the legalities as are USPS and LE.