What is the meaning of "Mah Ashiv la'Shem"?
Radak: What bestowal and gift can I give to Hashem to return all the Chesed that He did with me?
Why does it say "Kol Tagmulohi Alai"?
Radak: All that He bestowed on me is Alai, to return corresponding to it, if I can, for I did not have merit that it was proper for Him to do for me what He did. It was all Chesed from Him, like Yakov said "Katonti mi'Kol ha'Chasadim umi'Kol ha'Emes Asher Asisa Es Avdecha" (Bereishis 32:11). "Tagmulohi" is like Tagmulav; in Arame'ic, Alohi is like Alav (on him, in Leshon ha'Kodesh); Yadohi is like Yadav.
Mesilas Yesharim (Perek 22): This is like an Oni who receives a gift of Chesed. It is inevitable that he is embarrassed. The more Chesed he receives, the more he is ashamed. So it is with anyone whose eyes are open to see himself, when he reaches great attributes from Hashem.
Malbim: He revived me after this great despair. His great bestowal on me (saving me) is that I will be able to thank Him, publicize His grandeur and fulfill my vows - "Kos Yeshu'os Esa uv'Shem Hashem Ekra; Nedarai la'Shem Ashalem" (13-14).
