Why did Bil'am add the word "Lesiti Lahaloch Imachem"?
Rashi and Rashbam (in Pasuk 14): He was insinuating that Balak's current emissaries were not commensurate with his honor 1 and that Balak should therefore send more important dignitaries, 2 which he did. 3
Rashi: This teaches us that Bil'am was conceited and that he wanted to hide the fact that he was under Hashem's jurisdiction - to the extent that his Kavod outweighed the lives of millions of people whose lives hung in the balance. See Oznayim la'Torah.
Implying that Hashem had refused his request the first time in defense of his (Bil'am's) honor.
Following the words of his emissaries, who reiterating Bil'am's message when, in Pasuk 14, they too, added the word "Imanu" (Rashbam). See Ramban's objections to this explanation.
Ramban: In whose name he proudly spoke.
Ramban: And it was because Balak suspected him of expecting a higher wage than usual that he said to him (in Pasuk 37) "Am I not able to pay you whatever you ask!"
QUESTIONS ON RASHI
Rashi writes that Bil'am said "Imachem" to hint that they are below his dignity, since he was conceited. But he merely conveyed what Hashem told him, "Lo Selech Imahem"?
Moshav Zekenim: Rashi learns that he was conceited from the word "Lesiti", which is otherwise superfluous.