Let me know if "Have Spacesuit, Will Travel" shows up. I'm going to guess it hasn't aged as well, given its initial setting.
Visiting a new library in 5th grade, I found a book about a kid my age who entered a contest with cereal box-tops. Those of you under the age of 45 have no idea what that's about, but the early days of Broadcast TV in the 60's featured promotions with cereal companies, who often offered mail-in sweepstakes for fantastic prizes (to a kid.)
Your selection is the favorite of Melinda Snodgrass, the co-editor of the Wild Card series.