My world famous and soon to be a major motion picture talk on Purgatory compares it to physical therapy. It's true and regrettable, IMHO, that a lot of the language is either penal or monetary. But anyone who has ever dieted or exercised -- leave alone damaged a ligament -- almost naturally makes the leap from a "therapeutic" to an "accounting" metaphor. Last week's vacation must be "paid for" with this week's time on the elliptical or treadmill. This cupcake will cost me down the road.
If, because of a wound, I have wounded myself further by favoring the injured member and asking too much of others to compensate, there will be labor and pain to restore strength and freedom of motion. But the whole process is "happy" because it is "toward" strength and freedom, and THOUGH one works, one experiences the healing as grace.
It is entirely correct to say that Christ accomplished all of our salvation for us on the cross. But that does not settle the question of how this redemption is applied to us.