In her Dialogue, St. Catherine says that each of us chooses to live our lives by walking either on the Bridge, which is Christ, or on the river, which is the way without God, and in which one drowns. Both ways are full of fatigue, she says, but in choosing the Bridge, one experiences delight, and “every bitterness becomes sweet, and every burden light.”
She writes, speaking for God the Father,
For those who cross by the Bridge, being still in the darkness of the body, find light, and, being mortal, find immortal life, tasting, through love, the light of Eternal Truth which promises refreshment to him who wearies himself for Me…. He, therefore, is a fool indeed, who despises so great a good, and chooses rather to receive in this life, the earnest money of Hell, walking by the lower road with great toil, and without any refreshment or advantage.
In discussing this with my "cloistered brothers," a humorous but true image was conveyed. We haven't crossed the bridge. We sinners have to pay a toll to get over the bridge.
St. Catherine isn't the only one to use metaphors. The toll is penance. Another thought could be that the toll is grace. And if you combine the two, then the toll is the grace to pay the penance.