Samsung Galaxy Nexus (LTE)

I went to the Google Wallet website to see which websites accepted the feature. Among them were Buy.com and JR.com. However, when I clicked on the Google Checkout button, it says that my current method of payment is invalid. Why is this when I have $50 worth of credit in my Google Wallet? Anyone having this issue or am I just doing something wrong? Am I not actually able to use this online?