Why does it say "Ko Amar Hashem"?
Malbim: This refers to the first matter (do not rely on the false Nevi'im who say "Heichal Hashem?" - verse 4. It will not help you, for) Hashem does not want your Korbanos.
Why does it mention Olos and Shelamim, but not Chatas and Asham?
Radak: They did not admit their sins and repent from them. Also, they sinned b'Mezid, and there is no Korban for Mezid 1 . They brought Olos to give the appearance that they serve Hashem. They brought Shelamim to eat the meat, and show that they serve Hashem.
Indeed, Chatas is not for Mezid, but several Ashamos are even for Mezid, e.g. Nazir Tamei, Shifchah Charufah, Gezelos (he intentionally swore falsely), Me'ilah according to R. Meir, and some say, Asham Taluy (he knew that it is Safek Isur). (PF)
What is the meaning of "Oloseichem Sefu Al Zivcheihem"?
Rashi: Add your Olos to your Shelamim. Since they are not accepted, why should they be totally burned? You can offer them as Shelamim, and eat the meat!
Radak: This is not a command 1 . Rather, whatever you do (you can add the Olos to the Shelamim if you want), it will not help.
Malbim: Cease offering Olos and Zevachim to Hashem. Rather, eat the meat [as Chulin].
Radak: Sefu can be from the root Yasaf, or from Safah, like "Sefos Chatas Al Chatas" (Yeshayah 30:1), "Lisfos Od" (Bamidbar 32:14).
