diff --git a/services/nginx/app/objects/collected_order_invoices_o.php b/services/nginx/app/objects/collected_order_invoices_o.php
index ef119738..e1564c07 100644
--- a/services/nginx/app/objects/collected_order_invoices_o.php
+++ b/services/nginx/app/objects/collected_order_invoices_o.php
@@ -40,20 +40,29 @@ class collected_order_invoices_o extends db
         $db;
         // Sanitize the input
         $customer_number = $db->escape_string($customer_number);
-        $name = $db->escape_string($name);
-        $notes = $db->escape_string($notes);
-        $processor = $db->escape_string($processor);
+        if (!empty($name)) {
+            $name = $db->escape_string($name);
+        }
+        if (!empty($notes)) {
+            $notes = $db->escape_string($notes);
+        }
+        if (!empty($processor)) {
+            $processor = $db->escape_string($processor);
+        }
         // Require the customer number to be of a valid customer
         self::requireValidCustomer($customer_number);
         // Add the object
         $tmp_id = self::add_object([
             'customer_number' => (int)$customer_number,
             'name' => $name,
-            'notes' => $notes,
-            'processor' => $processor
+            'notes' => $notes
         ]);
         self::select((int)$tmp_id);
         self::requireSelected();
+        // Set the processor
+        if (!empty($processor)) {
+            $this->processor->set($processor);
+        }
         self::objectChanged();
         return $this;
     }
@@ -124,7 +133,104 @@ class collected_order_invoices_o extends db
             'created_at' => (string)$this->created_at->value(),
             'updated_at' => (string)$this->updated_at->value(),
             'closed_at' => (string)$this->closed_at->value(),
+            'orders' => $this->getOrders()
         ];
     }
 
+    /**
+     * Get the orders in the invoice collection
+     * @param bool $count If the count of orders should be returned instead of the orders
+     * @return array|int The list of orders in the invoice collection, or the count of orders if $count is true
+     * @throws Exception If the request was not successful
+     */
+    public function getOrders(bool $count = false): array|int
+    {
+        $orders = new orders_o();
+        $order_ids = $orders->getFieldsWhere(
+            [
+                'invoice_collection_id' => $this->id,
+                'deleted_at' => null
+            ],
+            ['id']
+        );
+        if ($count) {
+            return count($order_ids);
+        }
+        $result = [];
+        foreach ( $order_ids as $order_id ) {
+            $orders->select($order_id['id']);
+            $orders->requireSelected();
+            $result[] = $orders->asArray();
+        }
+        return $result;
+    }
+
+    /**
+     * Check if a customer has an open invoice collection
+     * @param int $customer_number The E-conomic customer number
+     * @return bool If the customer has an open invoice collection
+     */
+    public function hasOpenInvoiceCollection(int $customer_number): bool
+    {
+        $collections = self::getFieldsWhere(
+            [
+                'customer_number' => $customer_number,
+                'closed_at' => null,
+            ],
+            ['id']
+        );
+        return !empty($collections);
+    }
+
+    /**
+     * Get the open invoice collections for a customer
+     * @param int $customer_number The E-conomic customer number
+     * @return array The open invoice collections
+     * @throws Exception If the request was not successful
+     */
+    public function getOpenInvoiceCollections(int $customer_number): array
+    {
+        $collections = self::getFieldsWhere(
+            [
+                'customer_number' => $customer_number,
+                'closed_at' => null,
+            ],
+            ['id', 'name', 'notes', 'processor', 'external_id', 'created_at', 'updated_at', 'closed_at']
+        );
+        // Parse the results
+        $result = [];
+        foreach ( $collections as $collection ) {
+            $this->id = $collection['id'];
+            self::getObjectProperties();
+            self::requireSelected();
+            $result[] = self::asArray();
+        }
+        return $result;
+    }
+
+    /**
+     * @param int $customer_number The E-conomic customer number
+     * @return self The latest open invoice collection
+     * @throws Exception If no open invoice collections were found
+     * @throws Exception If the request was not successful
+     */
+    public function getLatestOpenInvoiceCollection(int $customer_number): self
+    {
+        $collections = self::getFieldsWhere(
+            [
+                'customer_number' => $customer_number,
+                'closed_at' => null,
+            ],
+            ['id', 'name', 'notes', 'processor', 'external_id', 'created_at', 'updated_at', 'closed_at'],
+        );
+        if (empty($collections)) {
+            throw new Exception('No open invoice collections found');
+        }
+        // Parse the results
+        $this->id = $collections[0]['id'];
+        self::getObjectProperties();
+        self::requireSelected();
+        return $this;
+    }
+
 }
\ No newline at end of file
diff --git a/services/nginx/app/objects/orders_o.php b/services/nginx/app/objects/orders_o.php
index 37a67735..e8d1e5e7 100644
--- a/services/nginx/app/objects/orders_o.php
+++ b/services/nginx/app/objects/orders_o.php
@@ -25,6 +25,7 @@ class orders_o extends db
     public object_property $created_at;
     public object_property $deleted_at;
     public stripe_module_orders_o $stripe_module_orders;
+    public object_property $invoice_collection_id;
 
     public function structure(): void
     {
@@ -64,8 +65,9 @@ class orders_o extends db
         $this->economic_module_orders = (new economic_module_orders())->getByOrderId($this->id);
         $this->stripe_module_orders = (new stripe_module_orders_o())->select($this->id);
         $this->deleted_at = new object_property($this->table, $this->id, 'deleted_at', 'timestamp', false);
+        $this->invoice_collection_id = new object_property($this->table, $this->id, 'invoice_collection_id', 'int', false);
     }
-    
+
 
     public function includeIncludes(): orders_o
     {
@@ -150,6 +152,7 @@ class orders_o extends db
             'reg_3' => $this->reg_3->value(),
             'created_at' => $this->created_at->value(),
             'deleted_at' => $this->deleted_at->value(),
+            'invoice_collection_id' => (int)$this->invoice_collection_id->value(),
         ];
     }
 
@@ -318,12 +321,35 @@ class orders_o extends db
 
             // Set the values of the object properties
             $this->getObjectProperties();
+            self::requireSelected();
+            self::assignToInvoiceCollection();
             return $this;
         } catch (Exception $e) {
             $response->error($e->getMessage());
         }
     }
 
+    /**
+     * Assign the order to an invoice collection
+     * @param int|null $invoiceCollectionId The invoice collection id, if not set, the default invoice collection id will be used.
+     * @throws Exception If the order is not selected
+     */
+    public function assignToInvoiceCollection(int $invoiceCollectionId = null): void
+    {
+        // If the invoice collection id is not set, get the default invoice collection id
+        self::requireSelected();
+        // Get the customer
+        $customer = new users_o();
+        $customer->getUserByCustomerNumber($this->customer_id->value());
+        $customer->requireSelected();
+        // Get the invoice collection id
+        $invoiceCollectionId = $invoiceCollectionId ?? $customer->getNewOrderInvoiceCollectionId();
+        // Assign the order to the invoice collection
+        $this->invoice_collection_id->set($invoiceCollectionId);
+        self::objectChanged();
+    }
+
+
     public function objectChanged(): void
     {
         // Since the orders object is not cached, there is no need to invalidate the cache
diff --git a/services/nginx/app/objects/users_o.php b/services/nginx/app/objects/users_o.php
index 21b5daf3..fc466916 100644
--- a/services/nginx/app/objects/users_o.php
+++ b/services/nginx/app/objects/users_o.php
@@ -399,15 +399,6 @@ class users_o extends db
         return $this->doesUserHaveAttribute('requiresReferenceNumber');
     }
 
-    /**
-     * If the customer should have an invoice per order (Otherwise, they will have a monthly invoice for all orders)
-     * @return bool
-     */
-    public function invoicePerOrder(): bool
-    {
-        return $this->doesUserHaveAttribute('invoiceAllOrdersIndividually');
-    }
-
     /**
      * If the customer is NOT allowed to purchase spot free washes
      * @return bool
@@ -872,4 +863,65 @@ class users_o extends db
         $user = self::getFieldsWhere(['customer_number' => $customerNumber], ['id']);
         return $user[0]['id'];
     }
+
+    /**
+     * Get the invoice collection ID, where the new order should be added to.
+     * This is based on the user's settings. (If the user has the attribute 'invoiceAllOrdersIndividually', the new order should be added to a new invoice collection)
+     * @return int The ID of the order invoice collection the new order should be added to.
+     * @throws Exception
+     */
+    public function getNewOrderInvoiceCollectionId(): int
+    {
+        // Check if the user has the attribute 'invoiceAllOrdersIndividually'
+        $invoice_collection = new collected_order_invoices_o();
+        if ($this->invoicePerOrder() || !self::hasOpenInvoiceCollection()) {
+            // Require the user to have a customer number above 0
+            if (is_null($this->customer_number->value()) || $this->customer_number->value() === 0) {
+                throw new Exception('The user does not have a customer number');
+            }
+            // The user has the attribute 'invoiceAllOrdersIndividually', create a new invoice collection
+            $invoice_collection->add((int)$this->customer_number->value());
+            return $invoice_collection->id;
+        }
+        // The user does not have the attribute 'invoiceAllOrdersIndividually', get the ID of the last invoice collection
+        return self::getOpenInvoiceCollection()->id;
+    }
+
+    /**
+     * If the customer should have an invoice per order (Otherwise, they will have a monthly invoice for all orders)
+     * @return bool
+     */
+    public function invoicePerOrder(): bool
+    {
+        return $this->doesUserHaveAttribute('invoiceAllOrdersIndividually');
+    }
+
+    /**
+     * Check if the user has an open invoice collection
+     * @return bool
+     */
+    public function hasOpenInvoiceCollection(): bool
+    {
+        // Check if the user has an open invoice collection
+        $invoice_collection = new collected_order_invoices_o();
+        return $invoice_collection->hasOpenInvoiceCollection($this->customer_number->value());
+    }
+
+    /**
+     * Get the open invoice collection of the user
+     * @return collected_order_invoices_o
+     * @throws Exception If the user does not have an open invoice collection
+     * @throws Exception If the user does not have a customer number
+     */
+    public function getOpenInvoiceCollection(): collected_order_invoices_o
+    {
+        // Require the user to have a customer number above 0
+        if (is_null($this->customer_number->value()) || $this->customer_number->value() === 0) {
+            throw new Exception('The user does not have a customer number');
+        }
+        self::requireSelected();
+        // Get the open invoice collection of the user
+        $invoice_collection = new collected_order_invoices_o();
+        return $invoice_collection->getLatestOpenInvoiceCollection($this->customer_number->value());
+    }
 }
\ No newline at end of file
diff --git a/services/nginx/app/routes/orderInvoicesRoute.php b/services/nginx/app/routes/orderInvoicesRoute.php
index 67de9112..04ebe788 100644
--- a/services/nginx/app/routes/orderInvoicesRoute.php
+++ b/services/nginx/app/routes/orderInvoicesRoute.php
@@ -47,7 +47,8 @@ class orderInvoicesRoute
                             'external_id' => (string)$collected_order_invoice['external_id'],
                             'closed_at' => $collected_order_invoice['closed_at'] ? (string)$collected_order_invoice['closed_at'] : null,
                             'updated_at' => (string)$collected_order_invoice['updated_at'],
-                            'created_at' => (string)$collected_order_invoice['created_at']
+                            'created_at' => (string)$collected_order_invoice['created_at'],
+                            'orders' => ((new collected_order_invoices_o())->select((int)$collected_order_invoice['id']))->getOrders(true)
                         ];
                     }
                 ));