Doesn't the idea of purgatory add to Christ's work on the Cross?
On the Cross, Jesus earned infinitely more saving grace than every human being who has ever lived or ever will live could possibly need for salvation. Purgatory is not for people who failed to be saved on earth. The souls in purgatory are saved, but need to be purged of their attachments to sin before entering heaven. It is not something added to Christ's death on the Cross. The merits applied to the souls in purgatory were also earned on the Cross by Jesus.
Is purgatory a second chance for people who aren't good enough to go to heaven, but too good to go to hell?
No. Purgatory is more like the lobby of heaven. The people in purgatory are saved, but need to be purged of their attachments to sin before entering heaven. Let me ask you, are you ever tempted to sin? … When you get to heaven will you still be tempted to sin? … So a change must take place between earth and heaven that removes all temptation to sin — that is purgatory.
Where's purgatory in the Bible?
In First Corinthians 3:15, Paul talks about Christians building on the foundation of Jesus. When the quality of their work is tested on Judgment Day, the work of the first man survives because he built with good materials. The work of the second man, though, was made with imperfect materials and is burned up and he "suffers loss," as Paul puts it. He himself, however, "will be saved, but only as through fire.”
Read more about purgatory in the Catechism of the Catholic Church (see #1030).
Isn't the Catholic teaching on indulgences an attempt to buy your way to heaven?
Indulgences do not involve forgiveness of sins or eternal punishment (i.e., hell). They involve temporal (or temporary) punishment due for sins that have already been forgiven. It is not enough to be forgiven for our sins, the harm caused by those sins must be repaired. By God’s grace, we may participate in this reparation through small sacrificial acts, made powerful by being connected to Christ’s Supreme Sacrifice on the Cross (cf. John 15:5; Col. 1:24). In the Bible we see that temporal punishment remains even after sins are forgiven. While David is forgiven by the Lord for his adultery, he must suffer the loss of his son (2 Sam. 12:13-14). Strictly speaking, we can merit nothing before God. Yet, out of generosity, God assigns merit to acts of piety and charity. This is the meaning of the biblical teaching that "love covers a multitude of sins" (1 Pet. 4:8). Jesus assures us that when we fast or carry out spiritual sacrifices in a spirit of love “your Father who sees in secret will reward you” (Matt. 6:18). The Bible also indicates one may be healed by the vicarious faith of others, such as the paralytic, who is healed and forgiven by Jesus through the faith of his friends (Matt. 9:1-8).