Does LE need a search warrant for a package a K9 alerted on at the post office or shipping company?

Yes. The K9 alert provides probable cause, which is the amount of proof necessary for a warrant. However, there is no exception to the warrant requirement (as opposed to a vehicle, for which the law provides an exception because of a vehicle’s inherent mobility). For a package, a warrant is needed. The probable cause for the warrant is the K9 alert.

Interestingly, if a mail or package service employee opens the package to inspect the contents and then calls LE, LE can start from that point and view the contents as presented by the employee (or have a K9 sniff it). This only works if there is no contact or agreement with LE to open packages.