Our little boy came up to his mother in th

问题：

Our little boy came up to his mother in the kitchen one evening while she was fixing supper, and handed her a piece of paper that he had been writing on. After his morn dried her hands on an apron, she read it, and this is what it said:
For cutting the grass $ 5.00
For cleaning up my room this week $ 1.00
For going to the store for you $ 0.50
Baby-sitting my kid brother while you went shopping $ 0.25
Taking out the garbage $ 1.00
For getting a good report card $ 5.00
For cleaning up and raking the yard $ 2.00
Total owed : $ 14.75
Well, I’ll tell you, his mother looked at him standing there expectantly, and boy, could I see the memories flashing through her mind. So she picked up the pen, turned over the paper he’d written on, and this is what she wrote:
For the nine months I carried you while you were growing inside me, no charge.
For all the nights that I’ve sat up with you, doctored and prayed for you, no charge.
For all the trying times, and all the tears that you’ve caused through the years, there’s no charge.
For all the nights that were filled with dread, and for the worries I knew were ahead, no charge.
For the toys, food, clothes, and even wiping your nose, there’s no charge, son.
When you ______, the cost of my love is no charge.
Well, friends, when our son finished reading what his mother had written, there were great big old tears in his eyes, and he looked straight up at his mother and said: "Mom, I sure do love you. "
And then he took the pen and in great big letters he wrote: "PAID IN FULL. "

What do you learn after reading the passage (within 20 words)