We went shopping to help a congregant! It is a mitzvah!
"It’s a mitzvah!” can refer to a beneficial act performed by another person. That is it's colloquial usage.
The Hebrew word mitzvah does not literally mean “a good deed” but rather means “commandment.” In fact, Jewish tradition understands exactly 613 mitzvot from the Hebrew Bible. Sepher HaChinuch gives one list and various other commentators give alternative ones, but they all count to 613.
We found everything that we needed and thanked the store rep. She was very very nice!