Why was my credit card charged US$1?

I joined Audible.com recently. I tried to buy an book from them but was rejected due to unsuitable geographic location. I tried to cancel it but found it impossible to cancel. Anyway, I emailed them indirectly to ask them to help me to cancel. Today, I was shocked to discover that US$1 has been deducted by audible.com. I did not buy anything from them. Anyone knows why?