feat: implement custom invoice generation functionality

system/controllers/invoices.php

@@ -0,0 +1,41 @@
+<?php
+// Advanced usage with custom parameters
+try {
+    $userId = 8; // Customer ID
+    $status = "Unpaid";
+    $invoiceNo = "INV-2023-00987";
+
+    // Manually provide invoice data
+    $invoiceData = [
+        'plan_name' => 'Premium Plan',
+        'price' => 49.99,
+        'routers' => 'router123',
+        // Add other required fields from tbl_transactions
+    ];
+
+    // Custom bills
+    $bills = [
+        'Additional Bandwidth' => 15.00,
+        'Support Fee' => 10.00,
+        'IP Address' => 5.00,
+        'Custom Service' => 25.00,
+        'Late Fee' => 5.00,
+        'Discount' => -10.00,
+    ];
+
+    $add_cost = 20;
+
+    $result = Invoice::sendInvoice(
+        userId: $userId,
+        invoice: $invoiceData,
+        bills: $bills,
+        status: $status,
+        invoiceNo: $invoiceNo
+    );
+
+    if($result) {
+        echo "Custom invoice sent! PDF generated at: system/uploads/invoices/invoice_{$invoiceNo}.pdf";
+    }
+} catch (Exception $e) {
+    echo "Invoice Error: " . $e->getMessage();
+}