diff --git a/src/controllers/Cart.php b/src/controllers/Cart.php
index 5f93ec9..aec8d85 100644
--- a/src/controllers/Cart.php
+++ b/src/controllers/Cart.php
@@ -7,7 +7,6 @@
 use Exception;
 use Steamy\Core\Controller;
 use Steamy\Core\Utility;
-use Steamy\Model\Mailer;
 use Steamy\Model\Order;
 use Steamy\Model\OrderProduct;
 use Steamy\Model\Product;
@@ -71,8 +70,6 @@ private function handleInvalidURL(): void
 
     private function handleCheckout(): void
     {
-        // TODO: write appropriate errors to Cart view instead of sending response code
-
         // check if user is logged in
         $signed_client = $this->getSignedInClient();
         if (!$signed_client) {
@@ -110,24 +107,35 @@ private function handleCheckout(): void
             $new_order->addLineItem($line_item);
         }
 
-        // save order
-        $success_order = false;
+        // attempt to save order. An exception will be generated in case of any errors.
         try {
             $success_order = $new_order->save();
-            http_response_code($success_order ? 201 : 400);
         } catch (Exception $e) {
             error_log($e->getMessage());
             http_response_code(500);
             echo json_encode(['error' => $e->getMessage()]);
+            return;
         }
 
-        // send confirmation email if order was successfully saved
-        if ($success_order) {
-            try {
-                $signed_client->sendOrderConfirmationEmail($new_order);
-            } catch (Exception $e) {
-                error_log($e->getMessage());
-            }
+        // if order was unsuccessfully saved without any exceptions generated
+        if (!$success_order) {
+            http_response_code(500);
+            echo json_encode(['error' => "Order could not be saved for an unknown reason."]);
+            return;
+        }
+
+        // send confirmation email
+        try {
+            $success_mail = $signed_client->sendOrderConfirmationEmail($new_order);
+        } catch (Exception $e) {
+            http_response_code(503);
+            echo json_encode(['error' => "Order was saved but email could not be sent: " . $e->getMessage()]);
+            return;
+        }
+
+        if (!$success_mail) {
+            http_response_code(503);
+            echo json_encode(['error' => "Order was saved but email could not be sent for an unknown reason."]);
         }
     }
 
diff --git a/src/models/Order.php b/src/models/Order.php
index 00cef30..1448a12 100644
--- a/src/models/Order.php
+++ b/src/models/Order.php
@@ -122,12 +122,8 @@ public function save(): bool
         $update_stock_stm = $conn->prepare($query);
 
         foreach ($this->line_items as $line_item) {
-            if (!empty($line_item->validate())) {
-                // line item contains invalid attributes
-                $conn->rollBack();
-                $conn = null;
-                throw new Exception("Invalid line item:" . json_encode($line_item));
-            }
+            // set order ID of line item
+            $line_item->setOrderID($new_order_id);
 
             // fetch product corresponding to line item
             $product = Product::getByID($line_item->getProductID());
@@ -139,6 +135,9 @@ public function save(): bool
                 throw new Exception("Product with ID " . $line_item->getProductID() . " does not exist");
             }
 
+            // set true unit price of line item
+            $line_item->setUnitPrice($product->getPrice());
+
             // get stock level for current product
             $stock_level = $store->getProductStock($product->getProductID());
 
@@ -146,15 +145,39 @@ public function save(): bool
                 // store does not have enough stock
                 $conn->rollBack();
                 $conn = null;
+
+                $error_message = <<< EOL
+                Store with ID $this->store_id has insufficient stock ($stock_level) for the following line item:
+                Product ID = {$line_item->getProductID()} and quantity = {$line_item->getQuantity()}.
+                EOL;
+
+                throw new Exception($error_message);
+            }
+
+            // validate line item
+            $line_item_errors = $line_item->validate();
+            if (!empty($line_item_errors)) {
+                // line item contains invalid attributes
+                $conn->rollBack();
+                $conn = null;
+
+                $line_item_info = json_encode($line_item->toArray());
+                $line_item_errors = json_encode($line_item_errors);
+
+                $error_message = <<< EOL
+                Invalid line item:
+                $line_item_info
+                
+                Errors:
+                $line_item_errors
+                EOL;
+
                 throw new Exception(
-                    "Store with ID " . $this->store_id
-                    . " has insufficient stock for product " . $line_item->getProductID()
+                    $error_message
                 );
             }
 
-            // insert into order_product table
-            $line_item->setOrderID($new_order_id);
-            $line_item->setUnitPrice($product->getPrice());
+            // insert line item into order_product table
 
             $success = $insert_line_item_stm->execute($line_item->toArray());
             if (!$success) {
@@ -192,14 +215,9 @@ public function save(): bool
      *
      * @param OrderProduct $orderProduct
      * @return void
-     * @throws Exception
      */
     public function addLineItem(OrderProduct $orderProduct): void
     {
-        $errors = $orderProduct->validate();
-        if (!empty($errors)) {
-            throw new Exception("Invalid line item: " . json_encode($errors));
-        }
         $this->line_items[] = $orderProduct;
     }
 
diff --git a/src/models/OrderProduct.php b/src/models/OrderProduct.php
index 0048327..2c79e13 100644
--- a/src/models/OrderProduct.php
+++ b/src/models/OrderProduct.php
@@ -20,6 +20,15 @@ class OrderProduct
     private int $quantity;
     private float $unit_price;
 
+    /**
+     * Create a new OrderProduct object
+     * @param int $product_id
+     * @param string $cup_size
+     * @param string $milk_type
+     * @param int $quantity
+     * @param float|null $unit_price If not set, the default $unit_price is -1.
+     * @param int|null $order_id If not set, the default $order_id is -1.
+     */
     public function __construct(
         int $product_id,
         string $cup_size,