Or HaChaim on Deuteronomy 33:18/אור החיים על דברים ל״ג:י״ח

1 שמח זבלון בצאתך, "Rejoice, O Zevulun, on your journeys;" Although as a general rule we are not supposed to rejoice as per Kohelet 2,2 ולשמחה מה זה עושה, "and of joy, what does it accomplish?" Moses says here that Zevulun goes to sea joyfully even though he leaves the halls of Torah study in order to engage in commerce. The reason he is entitled to do this is that the proceeds of his commerce are intended to enable Issachar to devote himself to uninterrupted Torah study.

2Another meaning of these words is that when a man sets out on an undertaking he does not know if he will be successful. Moses is giving Zevulun an assurance that he will be successful; this is why he emphasizes that he may rejoice even at the outset of his journey. He may consider himself as a שליח מצוה, someone to whom the performance of a מצוה has been entrusted.

3באהליך, "in your tents." This is a simile for Issachar. The reason Moses uses this simile is twofold. 1) It denotes the fact that any dwelling in this world is to be considered only a temporary dwelling; our whole terrestrial existence is transient. 2) It describes that the Shechinah forms a "tent" over Zevulun and Issachar. Even though Zevulun is preoccupied with commerce and does not spend his time in the אהל של תורה, the tent of Torah, he qualifies for the presence of the Shechinah no less than Issachar his partner. David alludes to this when he said in Psalms 85,14: "Justice goes before him as he sets out on his way."