She would have to drag it through the sun and sand if she went now.

But her curiosity was too strong to allow her to wait.

She must find out what was in that letter to Joyce.

If it were from Jack, there would be something in it about their plans for the summer; maybe a kodak picture of the shack in the pine woods near the mines, where they were to board.
If it were from Holland, there would be another interesting chapter of his experiences on board the training-ship.
Once as she trudged along the road, it occurred to her that the letter might be from her cousin Kate, the "witch with a wand," who had so often played fairy godmother to the family.

She might be writing to say that she had sent another box.