What's the meaning of "I'm slinging mad volume and fat stacking benjies"?

Recently I was watching the television show Breaking Bad. There’s a sentence of dialogue from season 2, episode 6 that confused me:

Jesse Pinkman: You got something for me?
Skinny Pete: Yeah, I found ’em.
Jesse Pinkman: Is this a “five” or an “S”?
Skinny Pete: Five, yo. No wait… S. No, no… yeah, five.
Jesse Pinkman: Yeah? Jesus, how the hell do you spell “street” wrong? S-T-R-E-A-T?
Skinny Pete: Hey, man, I’m slingin’ mad volume and fat stackin’ benjies, you know what I’m sayin’? I can’t be all about, like, spelling and sh-t.

I am really confused as to the meaning of the last sentence, especially the words slinging and stacking here.