function smo_add_to_cart_func($atts) {
    ob_start();
	$atts = shortcode_atts(
        array(
            'product_id' => '',
        ), 
        $atts, 
        'smo_add_to_cart'
    );
	$product_id = $atts['product_id'];
	if ( ! class_exists( 'WooCommerce' ) ) {
        return;
    }
    $product = wc_get_product( $product_id );
    if ( ! $product || ! $product->is_purchasable() || ! $product->is_in_stock() ) {
        return;
    }
	
    echo '<form action="' . esc_url( get_the_permalink($product_id) ) . '" method="post" class="cart">';
    echo '<button type="submit" name="add-to-cart" value="' . esc_attr( $product_id ) . '" class="single_add_to_cart_button button alt">Add to cart - '.$product->get_price_html().'</button>';
    echo '</form>';
?>
<?php
	$output = ob_get_contents();
    ob_end_clean();
    return $output;
}

add_shortcode('smo_add_to_cart', 'smo_add_to_cart_func');