Rashi: Because it did extra, this punishment will come on it. They denied (lacked appreciation for) the good. [Yisrael's patriarch] Avraham stood for (helped their patriarch) Lot - when he left Charan, when he came to Egypt, in Avraham's merit he was saved from the destruction [of Sedom], and he fought Amrafel and his colleagues (the three kings with him). They should have bestowed good to Avraham's seed! When Sancheriv exiled Reuven and Gad, they were mourning; Mo'av taunted them - why do you mourn? You return to your father's house! Was not Avraham from Ever ha'Nahar?! This is "Shamati Cherpas Mo'av v'Gidufei Bnei Amon" (Tzefanyah 2:8). Also, they helped Sancheriv who besieged Shomron for three years - "b'Shalosh Shanim ki'Shnei Sachir v'Niklah Kevod Mo'av" (16:14).

Radak, Malbim: [Mo'av will mourn and cry] because the multitude that it conquered and was appointed over, the enemies will take all of it to Nachal ha'Aravim. Radak - it should say Asher Asah; the word Asher is omitted. Also "la'Am Aleha" (Yeshayah 42:5) is like Asher Aleha; "ha'Derech Yelchu Vah" (Shemos 18:20) is like Asher Yelchu Vah. "Ba'avur Zeh Asah Li" (ibid. 12:8) is like Ba'avur Zeh Asher Asah Li.

Malbim: Since the river will dry up, and they cannot export via boat matters that the land grew more than they need.

Rashi citing Targum Yonasan: Their Techumim (outside of their cities) on the western sea Yisnesev. Rashi - this is an expression of appointment, like "u'Fkudas Elazar ben Aharon ha'Kohen" (Bamidbar Shemos 4:16). I.e. their land, which they were appointed over, will be taken from them.

Rashi: The appointment over them [will be Bavel; refer to 15:7:3:2].

Radak: Their money. Money is called Pekudah, for a man is appointed and a ruler over it. This is like Yisrah; it is repeated in different words.

Malbim: It is matters not found in Mo'av that they imported from elsewhere.

Rashi #1: The enemy will take them, i.e. the borders.

Rashi #2: [The enemy will exile them] to a valley of Aravim (willow trees), i.e. Bavel - "Al Aravim b'Sochah Talinu Kinoroseinu" (Tehilim 137:2).

Radak: [What they will take from the city] they will take to] the bank of a river close to Ir Mo'av ¹ ; there were willow trees on the river [bank], like "Arvei Nachal" (Vayikra 23:40).

Malbim: They will take this merchandise on Nachal Egypt. Melech Ashur, who conquered them, brought them via Nahar Egypt; that is the way from Mo'av to Ashur ² . (Or it is the river of Bavel, if Melech Bavel destroyed them.) The metaphor says that since their business ceased, and all their wealth went to the land of their enemy, it is as if through the river drying, the business was diverted to another river.