In 2 Corinthians 8-9, Paul deals extensively with what it means to practice grace giving. In fact, in these two chapters alone, the word "grace" is mentioned six times.
We learn from these chapters that grace giving is God-honoring. When everything is going well in your life and you give God the crumbs, you are not honoring God. But when everything is blowing up in your face and you give to God sacrificially, you do honor God—something He will never forget. Problems are a part of living, but don't use problems as an excuse to not practice generosity.
They had joy in the fact that God's grace was poured out upon them.