Cannot buy songs / only option is gift

I am using Itunes on a Windows computer. I have about 100 purchased songs on this computer already. I have been creating a wish list of new songs to buy over the past several months. I have money in my Itunes account from a gift card I redeemed, but no other payment method specified. In the past, I would simply click the purchase price of an item in my list and then click the purchase option. Today, the purchase option is not there. I have options to gift the song to someone else, post comment, link, etc. But there is no option to purchase and download to this computer. The computer must already be authorized because I can play previously purchased items. What is going on?