Auto Add Product to Cart on Visit in WooCommerce

The following code lets you add a product to the cart automatically:

/**
 * Automatically add product to cart on visit
 */
add_action( 'template_redirect', 'add_product_to_cart' );
function add_product_to_cart() {
	if ( ! is_admin() ) {
		$product_id = 64; //replace with your own product id
		$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->get_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 );
		}
	}
}

If you want to add a product automatically based on cart total, use this code:

/**
 * Add another product depending on the cart total
 */
add_action( 'template_redirect', 'add_product_to_cart' );
function add_product_to_cart() {
  if ( ! is_admin() ) {
		global $woocommerce;
		$product_id = 2831; //replace with your product id
		$found = false;
		$cart_total = 30; //replace with your cart total needed to add above item

		if( $woocommerce->cart->total >= $cart_total ) {
			//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->get_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 );
			}
		}
	}
}

Take a look at the post How to add products to cart via link.