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/*
 * goes in theme functions.php or a custom plugin
 **/
// add item to cart on visit depending on cart total value
add_action( 'init', 'add_product_to_cart' );
function add_product_to_cart() {
  if ( ! is_admin() ) {
		global $woocommerce;
		$product_id = 2831;
		$found = false;
		$cart_total = 30;

		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->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 );
			}
		}
	}
}