Use following code to prevent applying same discount many times (for each added product!)

add_filter( "wpo_prepare_item", function ( $item, $product ) {
                $cost_updated_manually = isset( $item['cost_updated_manually'] ) ? $item['cost_updated_manually'] : false;
                if ( ! $cost_updated_manually ) {
                    $item['item_cost'] = $product->get_price('');
                }
                return $item;
}, 10, 2 );