No rule for this one
No ending or word group predicts the article of Paket. Learn it as a pair and say it out loud: das Paket.
das Paket
It's das Paket. Paket is neuter, plural die Pakete.
Das Paket kommt morgen.
The package arrives tomorrow.
No ending or word group predicts the article of Paket. Learn it as a pair and say it out loud: das Paket.