What is the polite way to say that I have been helped in shop? Let's say I am in a shop, an assistant helped me, then he/she went away for other stuffs, maybe he/she was checking the inventory for me, or maybe I don't have any further questions and I just want to look around. Then another assistant come to me and ask if I need help. Most of time in I just say "No thank you." But I think there should be a more polite way to say that some one else already helped me, I am just waiting for him/her or I don't need further help. So in this kind of situation what should I say?

Question when ordering taco bell. Hello guys. I am a English learner. Yesterday I went to Taco Bell to order my lunch. I ordered a crunchwrap and then said I'd like to change "beef" to "steak", then the clerk asked me a question but I totally could not catch what her said, she repeated twice but I still don't understand, then she said "you want 1 dollar or what else" or something like that, I answered 1 dollar. Then we done. Eventually I found they charged me $1.2 to change my "beef" to "steak" in crunchwrap. My question is, what probably did she ask? and usually, what the clerk will say when I am trying to custom your food? Thanks.