Just the add cart line method from the cart manager

CartManager.cs

/// <summary>
        /// Adds the line item to cart.
        /// </summary>
        /// <param name="storefront">The storefront.</param>
        /// <param name="visitorContext">The visitor context.</param>
        /// <param name="inputModelList">The input model.</param>
        /// <returns>
        /// The manager response where the result is retuned indicating the success or failure of the operation.
        /// </returns>
        public virtual ManagerResponse<CartResult, bool> AddLineItemsToCart([NotNull] CommerceStorefront storefront, [NotNull] VisitorContext visitorContext, IEnumerable<AddCartLineInputModel> inputModelList)
        {
            Assert.ArgumentNotNull(inputModelList, "inputModelList");

            var cartResult = this.LoadCartByName(storefront.ShopName, storefront.DefaultCartName, visitorContext.UserId, false);
            if (!cartResult.Success || cartResult.Cart == null)
            {
                var message = StorefrontManager.GetSystemMessage(StorefrontConstants.SystemMessages.CartNotFoundError);
                cartResult.SystemMessages.Add(new SystemMessage { Message = message });
                return new ManagerResponse<CartResult, bool>(cartResult, cartResult.Success);
            }

            var lines = new List<CartLine>();
            foreach (var inputModel in inputModelList)
            {
                Assert.ArgumentNotNullOrEmpty(inputModel.ProductId, "inputModel.ProductId");
                Assert.ArgumentNotNullOrEmpty(inputModel.CatalogName, "inputModel.CatalogName");
                Assert.ArgumentNotNull(inputModel.Quantity, "inputModel.Quantity");

                var quantity = (uint)inputModel.Quantity;

                //// Special handling for a Gift Card
                if (inputModel.ProductId.Equals(storefront.GiftCardProductId, StringComparison.OrdinalIgnoreCase))
                {
                    inputModel.VariantId = inputModel.GiftCardAmount.Value.ToString("000", CultureInfo.InvariantCulture);
                }

                var cartLine = new CommerceCartLine(inputModel.CatalogName, inputModel.ProductId, inputModel.VariantId == "-1" ? null : inputModel.VariantId, quantity);
                cartLine.Properties["ProductUrl"] = inputModel.ProductUrl;
                cartLine.Properties["ImageUrl"] = inputModel.ImageUrl;
                //// UpdateStockInformation(storefront, visitorContext, cartLine, inputModel.CatalogName);      

                lines.Add(cartLine);
            }

            var cartCache = CommerceTypeLoader.CreateInstance<CartCacheHelper>();
            cartCache.InvalidateCartCache(visitorContext.GetCustomerId());

            var cart = cartResult.Cart as CommerceCart;
            var addLinesRequest = new AddCartLinesRequest(cart, lines);
            addLinesRequest.RefreshCart(true);

            // Set the custom properties here, get them from a cookie or any other mechanism
            addLinesRequest.Properties["applyVIPStatus"] = true;

            var addLinesResult = this.CartServiceProvider.AddCartLines(addLinesRequest);
            if (addLinesResult.Success && addLinesResult.Cart != null)
            {
                cartCache.AddCartToCache(addLinesResult.Cart as CommerceCart);
            }

            this.AddBasketErrorsToResult(addLinesResult.Cart as CommerceCart, addLinesResult);

            Helpers.LogSystemMessages(addLinesResult.SystemMessages, addLinesResult);
            return new ManagerResponse<CartResult, bool>(addLinesResult, addLinesResult.Success);
        }