WooCommerce Automatically add product to cart on site visit

gistfile1.php

/*
 * goes in theme functions.php or a custom plugin
 **/
// add item to cart on visit
add_action( 'template_redirect', 'add_product_to_cart' );
function add_product_to_cart() {
	if ( ! is_admin() ) {
		$product_id = 64;
		$found = false;
		//check if product already in cart
		if ( sizeof( WC()->cart->get_cart() ) > 0 ) {
			foreach ( WC()->cart->get_cart() as $cart_item_key => $values ) {
				$_product = $values['data'];
				if ( $_product->id == $product_id )
					$found = true;
			}
			// if product not found, add it
			if ( ! $found )
				WC()->cart->add_to_cart( $product_id );
		} else {
			// if no products in cart, add it
			WC()->cart->add_to_cart( $product_id );
		}
	}
}

Thanks @sc0ttkclark I don't understand why WooCommerce official documentation is still showing outdated code.

It wasted my time, thank you for posting the link.

Is there a way to have it only add the product to the cart if a specific coupon code is applied?

WooCommerce raises a warning about accessing ID directly. It needs this change.

if ( $_product->get_id() == $product_id )

Is it possible to have simliar funcionality but for condition: “Add product to cart when user choose speicified shipping”

I would like to add a cardboard packaging for every order shipped by post.
It is neccessary for proper calculation of total order weight and shipping cost.