Example of adding custom details to buyCRED purchases.

Add Marker to buyCRED Purchases

/**
 * Step 1 - Create custom Shortcode
 * Creates a custom buyCRED shortcode that includes
 * the marker attribute that is passed on to the gateway.
 * @version 1.0
 */
add_shortcode( 'mycred_custom_buy', 'mycred_render_custom_buy_shortcode' );
function mycred_render_custom_buy_shortcode( $atts, $title = 'Buy Points' ) {
	if ( ! function_exists( 'mycred' ) ) return 'myCRED is not installed';

	extract( shortcode_atts( array(
		'gateway' => '',
		'amount'  => '',
		'gift_to' => false,
		'marker'  => '',
		'class'   => 'mycred-buy-link button large custom',
		'login'   => 'Please login to buy points'
	), $atts ) );

	// Load myCRED
	$mycred = mycred();

	// If we are not logged in
	if ( ! is_user_logged_in() ) return '<div class="mycred-buy login">' . $mycred->template_tags_general( $login ) . '</div>';

	$url = get_bloginfo( 'url' ) . '/';
	$args = array(
		'mycred_buy' => $gateway,
		'amount'     => $mycred->number( $amount ),
		'token'      => wp_create_nonce( 'mycred-buy-creds' ),
		'marker'     => $marker
	);

	// Classes
	$classes = explode( ' ', $class );
	if ( empty( $classes ) )
		$classes = array( 'mycred-buy-link', 'button large', 'custom' );
	$classes[] = $gateway;

	$title = $mycred->template_tags_general( $title );

	// Element to return
	$element = '<a href="' . add_query_arg( $args, $url ) . '" class="' . implode( ' ', $classes ) . '" title="' . $title . '">' . $title . '</a>';

	return $element;
}

/**
 * Step 2 - Inject Marker
 * Next we inject our marker into the data sent to the gateway.
 * You would need to add a filter for each gateway you want this to support.
 * filter: mycred_{gateway id}_extra. Replace gateway with the gateway name.
 * Replace - with _ and remember to always return a result!
 * @version 1.0
 */
add_filter( 'mycred_paypal_standard_extra', 'mycred_inject_marker_for_gateway', 10, 6 );
function mycred_inject_marker_for_gateway( $extra, $cost, $from, $to, $gateway_prefs, $mycred ) {
	// Only add marker if it is set and not empty
	if ( isset( $_REQUEST['marker'] ) && ! empty( $_REQUEST['marker'] ) )
		return sanitize_text_field( $_REQUEST['marker'] );

	return $extra;
}

/**
 * Step 3 - Insert a custom payment column
 * Finally we will insert a custom column on the payment log to
 * show if a marker has been used for this payment.
 * @version 1.0
 */
add_filter( 'mycred_buycred_log_columns', 'mycred_adjust_purchase_log_columns' );
function mycred_adjust_purchase_log_columns( $columns ) {
	$columns['column-marker'] = 'Marker';
	return $columns;
}
add_action( 'mycred_payment_log_column-marker', 'mycred_payment_column_marker' );
function mycred_payment_column_marker( $log_entry ) {
	$data = maybe_unserialize( $log_entry->data );
	if ( isset( $data['sales_data'] ) ) {
		list ( $buyer_id, $payer_id, $amount, $cost, $currency, $token, $other ) = explode( '|', $data['sales_data'] );
		
		// Marker is saved under "other" last in the array.
		echo $other;
	}
	else {
		echo 'n/a';
	}
}