Add custom meta to export

give-export.php

<?php
/**
 * Add custom metadata to the columns for export
 *
 * @param array $data    Current data in the export
 * @param array $payment Array containing payment metadata
 *
 * @return array $data - Modified data
 */
function add_data_to_export( $data, $payment ) {
	// This assumes meta value is already added to the donation. You could call another function here to get the value.
	// If the meta is a part of the donation form you can get it using give_get_payment_form_id( id );
	$data['meta_name'] = ( isset( $payment->payment_meta['meta_name'] ) ) ? $payment->payment_meta['meta_name'] : 'N/A';
	return $data;
}
add_filter( 'give_export_donation_data', 'add_data_to_export' );

/**
 * Changes the column labels for custom metadata on export
 *
 * @param array $cols    Actual column names in the export
 * @param array $columns Array containing info about which columns have been selected for export
 *
 * @return array $cols   Columns with modified names
 */
function change_export_column_names( $cols, $columns ) {
	$cols['meta_name'] = 'Column Title';
	return $cols;
}
add_filter( 'give_export_donation_get_columns_name', 'change_export_column_names', 99, 2 );

/**
 * Add HTML to the CSV on the admin CSV export page
 *
 * @since v1.0
 *
 * @return void
 */
function donation_extra_fields() {
	?>
	<li class="give-export-option-start">
		<label for="give-export-donation-meta-name">
			<input type="checkbox" checked
					name="give_give_donations_export_option[meta_name]"
					id="give-export-donation-meta-name"><?php _e( 'Label Name', 'give' ); ?>
		</label>
	</li>
	<?php
}
add_action( 'give_export_donation_standard_payment_fields', 'donation_extra_fields' );