下面是一个如何根据自定义字段值对帖子进行排序的想法，我还提供了一些更改/删除自定义字段值的函数。

<?php
$args = array(
    \'post_type\' => \'card\', // custom post type name - card
    \'meta_query\' => array(
        \'relation\' => \'AND\', // return post with meta-field key status = ununsed & amount = 10
        array(
            \'key\' => \'status\',
            \'value\' => \'unused\',
        ),
        array(
            \'key\' => \'amount\',
            \'value\' => 10           // Array usage to show mulitple values- \'value\' => array( 10, 20 ) 
        )
    )
);
$the_query = new WP_Query($args);
//this will show list of all available cards
while ( $the_query->have_posts() ) : $the_query->the_post();

if(/* condition paid */) {
$number = get_post_meta($post->ID, \'number\', true);
echo $number; // or send email then

//change status from unused to used
update_post_meta($post_id, number, used, $number);

//delete amount custom field
delete_post_meta($post_id, amount, 10);

} else {
//do stuff if not paid
}
endwhile;
?>