What should they pay?
Rashi: Their vows.
Who are "Kol Sevivav"?
Rashi: All who hear of this salvation [from Sancheriv]. And so occurred - "Yihyu Chamesh Arim b'Eretz Mitzrayim Medabros Sefas Kena'an v'Nishba'os la'Shem Tzevakos" (Yeshayah 19:18).
Radak: All the nations around Eretz Yisrael. Since Hashem dwells in Eretz Yisrael, they are called Sevivav.
Why will they bring gifts?
Rashi: It is because when He desires, He holds back the pride of rulers (verse 13).
Malbim: The real source of fear is Hashem. All feared Ashur's power because Hashem apportioned to it part of the fear of Him. Why should they bring gifts to Ashur? Rather, they should bring to the source of fear!
What is "l'Mora"?
Radak: It is Hashem, who is awesome - "v'Hu Mora'achem" (Yeshayah 8:13).
