Even though Clay Aiken lost on ''American Idol,'' he's beating Ruben Studdard on one front: the sales chart on Amazon.com. Thanks to preorders, Clay's forthcoming single, ''This Is the Night/On the Wings of Love,'' is the best-selling music item in the entire online store. Meanwhile, Ruben's ''Flying Without Wings/Superstar'' comes in at No. 7, behind such established acts as Norah Jones and Staind (see the chart). Apparently, this doesn't mean that Clay has more voting fans than Ruben -- just more fans with Internet access and credit cards. But, really, there ARE no losers here. Ruben's CD single will hit stores June 3, while Clay's will drop June 10.