diff --git a/assets/js/billing.js b/assets/js/billing.js
index 282d32e..4c2f680 100644
--- a/assets/js/billing.js
+++ b/assets/js/billing.js
@@ -1,13 +1,33 @@
 function initBillingPage(projectId) {
-    document.querySelectorAll('.billing-amount').forEach(input => {
-        input.addEventListener('blur', function() {
-            const month = this.dataset.month;
-            const amount = this.value;
+    const saveBtn = document.getElementById('save-billing');
+    const billingAmountInputs = document.querySelectorAll('.billing-amount');
+    const totalBillingCell = document.getElementById('total-billing');
+
+    function updateTotalBilling() {
+        let total = 0;
+        billingAmountInputs.forEach(input => {
+            total += parseFloat(input.value) || 0;
+        });
+        totalBillingCell.textContent = `€${total.toLocaleString('en-US', { minimumFractionDigits: 2, maximumFractionDigits: 2 })}`;
+    }
+
+    billingAmountInputs.forEach(input => {
+        input.addEventListener('keyup', updateTotalBilling);
+    });
+
+    if (saveBtn) {
+        saveBtn.addEventListener('click', function() {
+            const billingData = [];
+            billingAmountInputs.forEach(input => {
+                billingData.push({
+                    month: input.dataset.month,
+                    amount: input.value
+                });
+            });
 
             const formData = new FormData();
             formData.append('projectId', projectId);
-            formData.append('month', month);
-            formData.append('amount', amount);
+            formData.append('billingData', JSON.stringify(billingData));
 
             fetch('save_billing.php', {
                 method: 'POST',
@@ -16,9 +36,9 @@ function initBillingPage(projectId) {
             .then(response => response.json())
             .then(data => {
                 if (data.success) {
-                    // Maybe show a small success indicator
+                    alert('Billing data saved successfully!');
                 } else {
-                    alert('Error saving billing amount: ' + data.error);
+                    alert('Error saving billing data: ' + data.error);
                 }
             })
             .catch(error => {
@@ -26,5 +46,8 @@ function initBillingPage(projectId) {
                 alert('An unexpected error occurred.');
             });
         });
-    });
+    }
+
+    // Initial calculation
+    updateTotalBilling();
 }
diff --git a/billing.php b/billing.php
index e404f20..9805ab8 100644
--- a/billing.php
+++ b/billing.php
@@ -63,6 +63,7 @@ try {
                 <h1 class="h2">Billing - <?php echo htmlspecialchars($project['name']); ?></h1>
                 <div class="header-actions">
                     <a href="project_details.php?id=<?php echo $projectId; ?>" class="btn btn-secondary">Return</a>
+                    <button id="save-billing" class="btn btn-primary">Save</button>
                     <a href="export_billing.php?id=<?php echo $projectId; ?>" class="btn btn-secondary">Export to Excel</a>
                 </div>
             </div>
@@ -84,16 +85,20 @@ try {
                                     $currentMonth->modify('+1 month');
                                 }
                                 ?>
+                                <th style="width: 150px;">Total</th>
                             </tr>
                         </thead>
                         <tbody>
                             <tr>
                                 <td>Billing</td>
                                 <?php
+                                $totalBilling = 0;
                                 foreach ($billingData as $billingMonth) {
+                                    $totalBilling += $billingMonth['amount'];
                                     echo '<td><input type="number" class="form-control billing-amount" data-month="' . $billingMonth['month'] . '" value="' . htmlspecialchars($billingMonth['amount']) . '"></td>';
                                 }
                                 ?>
+                                <td id="total-billing" class="fw-bold">&euro;<?php echo number_format($totalBilling, 2); ?></td>
                             </tr>
                         </tbody>
                     </table>
diff --git a/export_billing.php b/export_billing.php
index 733e481..cb2110c 100644
--- a/export_billing.php
+++ b/export_billing.php
@@ -26,9 +26,12 @@ try {
 
     // Header row
     $header = ['Billing'];
+    $totalBilling = 0;
     foreach ($billingData as $row) {
         $header[] = date("M Y", strtotime($row['month']));
+        $totalBilling += $row['amount'];
     }
+    $header[] = 'Total';
     fputcsv($output, $header);
 
     // Data row
@@ -36,6 +39,7 @@ try {
     foreach ($billingData as $row) {
         $data[] = $row['amount'];
     }
+    $data[] = $totalBilling;
     fputcsv($output, $data);
 
     fclose($output);
diff --git a/save_billing.php b/save_billing.php
index 4481695..961d9b2 100644
--- a/save_billing.php
+++ b/save_billing.php
@@ -6,31 +6,43 @@ $response = ['success' => false, 'error' => 'Invalid request'];
 
 if ($_SERVER['REQUEST_METHOD'] === 'POST') {
     $projectId = $_POST['projectId'] ?? null;
-    $month = $_POST['month'] ?? null;
-    $amount = $_POST['amount'] ?? null;
+    $billingDataJson = $_POST['billingData'] ?? null;
 
-    if ($projectId && $month && $amount !== null) {
-        try {
-            $pdo = db();
-            $stmt = $pdo->prepare("UPDATE billingMonthly SET amount = :amount WHERE projectId = :projectId AND month = :month");
-            $stmt->execute([
-                ':amount' => $amount,
-                ':projectId' => $projectId,
-                ':month' => $month
-            ]);
+    if ($projectId && $billingDataJson) {
+        $billingData = json_decode($billingDataJson, true);
+
+        if (is_array($billingData)) {
+            try {
+                $pdo = db();
+                $stmt = $pdo->prepare("UPDATE billingMonthly SET amount = :amount WHERE projectId = :projectId AND month = :month");
+
+                $pdo->beginTransaction();
+                foreach ($billingData as $item) {
+                    $stmt->execute([
+                        ':amount' => $item['amount'],
+                        ':projectId' => $projectId,
+                        ':month' => $item['month']
+                    ]);
+                }
+                $pdo->commit();
 
-            if ($stmt->rowCount() > 0) {
                 $response['success'] = true;
                 unset($response['error']);
-            } else {
-                $response['error'] = 'No record found to update.';
+
+            } catch (PDOException $e) {
+                if ($pdo->inTransaction()) {
+                    $pdo->rollBack();
+                }
+                $response['error'] = 'Database error: ' . $e->getMessage();
+            } catch (Exception $e) {
+                $response['error'] = 'An unexpected error occurred: ' . $e->getMessage();
             }
-        } catch (PDOException $e) {
-            $response['error'] = 'Database error: ' . $e->getMessage();
+        } else {
+            $response['error'] = 'Invalid billing data format.';
         }
     } else {
         $response['error'] = 'Missing required fields.';
     }
 }
 
-echo json_encode($response);
+echo json_encode($response);
\ No newline at end of file
diff --git a/save_override.php b/save_override.php
index 6291136..8503930 100644
--- a/save_override.php
+++ b/save_override.php
@@ -93,5 +93,11 @@ if ($_SERVER['REQUEST_METHOD'] === 'POST') {
     }
 }
 
-echo json_encode($response);
+try {
+    echo json_encode($response, JSON_THROW_ON_ERROR);
+} catch (JsonException $e) {
+    error_log('JSON encoding error: ' . $e->getMessage());
+    http_response_code(500);
+    echo json_encode(['success' => false, 'error' => 'Internal server error during JSON encoding.']);
+}
 exit;
\ No newline at end of file