When we're out of food, we go (grocery) shopping.

Uly, your translation corresponds to "Когда у нас ЗАКОНЧИТСЯ еда, мы КУПИМ..." - an open condition with the non-recurring situation.
The original sentence is "Когда у нас ЗАКАНЧИВАЕТСЯ еда, мы СОВЕРШАЕМ покупки." The meaning is "EVERY TIME we run out of food, we go and buy groceries."
By the way, the original sentence sounds strange to me: "Мы совершаем покупки" means "We go shopping" without specifying any food. It sounds like "When we are hungry with no food in-store, we go buying shoes, clothes, perfume, etc."

I would say: "Когда у нас заканчивается еда, мы идём покупать продукты."
Note: in everyday life, "продукты" refer to what we buy in a food store.

Actually WILL in my translation can refer to a future concrete action, or to a generality - this is what we always do. But I’ll add another translation that’s more general.