Why did Billy decide to take the cup home in place of getting it engraved right away?

Billy thought about his sister awaiting their return and decided it was time to go home.

Looking at the cup, it seemed that far down in the gleaming shadows I could see two wide blue eyes glued to a windowpane. I knew that my little sister was watching the road and waiting for our return. Looking back at the judge, I said, "If you don't mind, I'll take it with me. My grandfather can send it in for me."