Automatically add product to cart, on Woocommerce.

functions.php

<?php
// Add item to cart on visit
add_action( 'init', 'add_product_to_cart' );
function add_product_to_cart() {
	if ( ! is_admin() ) {
		global $woocommerce;
		$product_id = THE_PRODUCT_ID;
		$found = false;
		//check if product already in cart
		if ( sizeof( $woocommerce->cart->get_cart() ) > 0 ) {
			foreach ( $woocommerce->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 )
				$woocommerce->cart->add_to_cart( $product_id );
		} else {
			// if no products in cart, add it
			$woocommerce->cart->add_to_cart( $product_id );
		}
	}
}
?>