It's a line from a song Called "milkshake". The word milkshake refers to handjobs.
That's why its called a "milk" "shake". Once you replace the word milkshake with handjob in the song it's alot easier to
understand. This is also why she says, " Damn right, it's
better than yours. I could teach you, but I'd have to charge."in this song the girl is boasting about how good she handjobs are