it's the least I can do idiom
尽一点微薄之力而已
a polite answer to someone who thanks you, usually when you feel you should do more to help
"Thanks for cleaning up" "It's the least I can do, seeing as I'm staying here rent-free." “谢谢你替我打扫卫生。”“我住在这里你连房租都没收，我做这点事算不了什么。”