Set Fixed Donation Amount

/**

 * Automatically set the donation amount and recurring

 * period for donations.

 *

 * @param   array $fields

 * @return  array

 */

function ed_charitable_set_donation_amount( $fields ) {

    unset( $fields['donation_fields'] );

    // The amount you would like to donate.

    $amount = 5;

    $fields['donation_amount'] = array(

        'type'     => 'hidden',

        'priority' => 0.1,

        'value'    => $amount,

    );

    return $fields;

}

add_filter( 'charitable_donation_form_fields', 'ed_charitable_set_donation_amount', 20 );