WalMart's site has the "AT&T Z990 Avail Prepaid Android GoPhone, Black" $124.88 but also has the "AT&T GoPhone Avail Smartphone, Black" for $129.99. What is the difference? The former lists all kinds of music functions in the description but they sound like the same model. Also, I hear that in WalMart they sell a package containing the Avail packed with some sort of extra (charging?) device for $129.88. What is the device?