One of the frustrating things for me with Google Books (aside from the search functionality) is that you cannot combine credits with other forms of payment. So, I have the $5 off promotion plus $5 of credits, but I cannot buy the book I want because it's $10.33 with tax. Google won't allow you to just pay the extra $0.33, but instead you cannot use any of your credits and must pay the full $5.33 difference.
I'm holding out in hopes they reward us with one more $1 credit before the end of the year.