Hi! Judging from your other posts that you're probably hearing English spoken in Massachusetts, I can definitely confirm that your understanding in this example is 100 percent correct.

Hello. To me, replying by "all set" would sound natural here only if what "A" is carrying are things that she needs go somewhere, or to do something she's about to do. It has more the meaning of "I'm ready" ("I'm all set (to go), or "I'm all set (to do...whatever she's planning to do). Maybe something like "一切准备停当。" in Chinese. Good luck !