'-는 김에' means "since one does something" It has nuance of "make good use of opportunity and do something" For example, 은행에 가는 김에 장을 봤다 Since I was going to the bank, I did grocery shopping. 말 나온 김에 부탁 하나 할게요 Since[Now that] you've brought it up, I will ask you a favor.

는 김에 is to describe that while someone is already doing an action, the person could get another opportunity to do the next behavior . ex) While cooking soup, I also cooked yours. 수프를 요리하는 김에 니 수프도 요리했어. While you do dish washing, could you bring this cup? 설거지 하는 김에. 이 컵도 가져가줄래?

일단 "겸사겸사"로 번역해 보았습니다. 그리고 그를 번역하게했더니 "might as well" 이 나오더군요. 즉, 시장에 간 김에(겸사 겸사) 씨리얼 한 박스 사와라. 는 If you were going to the market, you might as well buy a box of serial for me. 어때요?

"While you are at it~'' Please polish my shoes too while you are at it.