diff --git a/BTCPayServer.Plugins.Grin/Services/GrinPaymentMonitorService.cs b/BTCPayServer.Plugins.Grin/Services/GrinPaymentMonitorService.cs
index e61ec25..94ae159 100644
--- a/BTCPayServer.Plugins.Grin/Services/GrinPaymentMonitorService.cs
+++ b/BTCPayServer.Plugins.Grin/Services/GrinPaymentMonitorService.cs
@@ -261,22 +261,7 @@ private async Task DispatchWebhookAsync(GrinInvoice invoice, GrinStoreSettings s
 
         try
         {
-            var amountGrin = invoice.AmountNanogrin / 1_000_000_000m;
-            var payload = new
-            {
-                @event = eventType,
-                invoice = new
-                {
-                    id = invoice.Id,
-                    status = invoice.Status.ToString(),
-                    amount = amountGrin,
-                    metadata = new
-                    {
-                        session_id = invoice.SessionId ?? "",
-                        medusa_cart_id = invoice.OrderId ?? "",
-                    }
-                }
-            };
+            var payload = GrinWebhookPayload.Build(invoice, eventType);
 
             var json = JsonSerializer.Serialize(payload);
             var content = new StringContent(json, Encoding.UTF8, "application/json");
diff --git a/BTCPayServer.Plugins.Grin/Services/GrinService.cs b/BTCPayServer.Plugins.Grin/Services/GrinService.cs
index 3f758ab..ee0dddb 100644
--- a/BTCPayServer.Plugins.Grin/Services/GrinService.cs
+++ b/BTCPayServer.Plugins.Grin/Services/GrinService.cs
@@ -204,24 +204,7 @@ public async Task DispatchWebhook(GrinStoreSettings settings, GrinInvoice invoic
 
         try
         {
-            var payload = new
-            {
-                @event = eventType,
-                invoiceId = invoice.Id,
-                storeId = invoice.StoreId,
-                invoice = new
-                {
-                    id = invoice.Id,
-                    status = invoice.Status.ToString(),
-                    amount = invoice.AmountNanogrin / 1_000_000_000m,
-                    confirmations = invoice.Confirmations,
-                    metadata = new
-                    {
-                        session_id = invoice.SessionId ?? "",
-                        order_id = invoice.OrderId ?? "",
-                    },
-                },
-            };
+            var payload = GrinWebhookPayload.Build(invoice, eventType);
 
             var json = System.Text.Json.JsonSerializer.Serialize(payload);
             var body = System.Text.Encoding.UTF8.GetBytes(json);
diff --git a/BTCPayServer.Plugins.Grin/Services/GrinWebhookPayload.cs b/BTCPayServer.Plugins.Grin/Services/GrinWebhookPayload.cs
new file mode 100644
index 0000000..6227ad8
--- /dev/null
+++ b/BTCPayServer.Plugins.Grin/Services/GrinWebhookPayload.cs
@@ -0,0 +1,40 @@
+using BTCPayServer.Plugins.Grin.Data;
+
+namespace BTCPayServer.Plugins.Grin.Services;
+
+/// <summary>
+/// Single source of truth for the webhook JSON payload, shared by both dispatch
+/// paths — <see cref="GrinService.DispatchWebhook"/> (checkout / broadcast →
+/// InvoiceProcessing) and <see cref="GrinPaymentMonitorService"/> (settled /
+/// invalid / expired). Previously each path built its own anonymous object and
+/// they had drifted: the monitor omitted the top-level <c>invoiceId</c>/
+/// <c>storeId</c> and <c>confirmations</c>, and used <c>metadata.medusa_cart_id</c>
+/// while the checkout path used <c>metadata.order_id</c>. Consumers that handle
+/// both broadcast and settlement events therefore had to special-case two
+/// shapes (and any keyed only on <c>order_id</c> silently missed settlements).
+///
+/// To stay backward compatible, the order reference is emitted under BOTH
+/// <c>order_id</c> and <c>medusa_cart_id</c> (same value).
+/// </summary>
+public static class GrinWebhookPayload
+{
+    public static object Build(GrinInvoice invoice, string eventType) => new
+    {
+        @event = eventType,
+        invoiceId = invoice.Id,
+        storeId = invoice.StoreId,
+        invoice = new
+        {
+            id = invoice.Id,
+            status = invoice.Status.ToString(),
+            amount = invoice.AmountNanogrin / 1_000_000_000m,
+            confirmations = invoice.Confirmations,
+            metadata = new
+            {
+                session_id = invoice.SessionId ?? "",
+                order_id = invoice.OrderId ?? "",
+                medusa_cart_id = invoice.OrderId ?? "",
+            }
+        }
+    };
+}