Updates to Product Model, product updates, enable pricing update, improved formating of product services

app/Models/Product/Phone.php

@@ -5,6 +5,7 @@ namespace App\Models\Product;
 use Illuminate\Support\Collection;
 
 use App\Interfaces\ProductItem;
+use App\Models\Invoice;
 use App\Models\Service\Phone as ServicePhone;
 use App\Models\Supplier\Phone as SupplierPhone;
 
@@ -42,6 +43,8 @@ final class Phone extends Type implements ProductItem
 	// The model that is referenced when this product is ordered
 	protected string $order_model = ServicePhone::class;
 
+	// When comparing billing/pricing/charging, what metric to normalise to
+	const DefaultBill = Invoice::BILL_MONTHLY;
 	// The model that the supplier supplies
 	const SupplierModel = SupplierPhone::class;