Why does it say "Shilchu Char Moshel Eretz"?
Rashi: Do not be haughty. You know that your ruler, Meisha Melech Mo'av, used to pay Melech Yisrael 100,000 Charim 1 and 100,000 rams (Melachim II, 3:4) [each year]. Char is your ruler.
Radak #1: Your king used to pay a tax to Melech Yisrael, including Charim (refer to 16:1:1:1). In the days of Achav, he rebelled and stopped paying. This evil will not come upon you if you will send the Charim!
Radak #2: Shilchu refers to the future. Now your kingship will be lowered; Melech Ashur will destroy you and your pride will depart. The tax that you used to send to Melech Yisrael, send it to Har Bas Tziyon, to Chizkiyah Melech Yehudah, who will be a Tzadik and a strong king. Be submissive to him, and pay tax to him!
Malbim: After the prophecy of the fall of Mo'av, he counsels them how to be saved. Your only salvation is to send to Yerushalayim and request to let you live and hide in Eretz Yehudah. The Navi promises that Bnei Yehudah will have mercy on them, and agree. However, he sees that Mo'av, amidst pride, will not request; they will go from evil to evil. Char is a fast messenger. Moshel Eretz (the king himself) should be the messenger.
Charim are fattened bulls (Targum Yonasan) or fat lambs (Metzudas Tziyon).
What is "Sela Midbarah"?
Radak, Malbim: It is the capitol of Mo'av.
Why does it mention "Har Bas Tziyon"?
Radak: It is the capitol of Yisrael's kingship. Since there was no king in Shomron, you should have sent to Tziyon, to Melech Yehudah.
Malbim: It is the capitol of Yehudah. You should request that they accept you in their land, and there will be a remnant of you.
