I just came across a book at the local library titled "Cash, Credit Cards or Checks: A Book About Payment Methods" by Nancy Loewen. The ideal age for this book is probably 8 or older. It's a story of two kids and their dad on a shopping trip to prepare for school. It very clearly explains about the many different ways you can use to purchase items and how they work. It also has a few tips about saving and making wise purchases.
There are other books in the same series that I will be checking out in the future.