docs: add payment-failed documentation

Clarifies that payment-failed is an Action (notification), NOT an order Status.

Key points documented:
- Order remains in settled-hold-invoice status during payment retries
- Mostro retries automatically based on payment_attempts and interval
- If all retries fail, Mostro sends add-invoice for buyer to provide new invoice
- Clients should NOT create a PaymentFailed order status

Also updated release.md to reference the new payment_failed.md page.

Author: MostronatorCoder[bot]

src/SUMMARY.md

@@ -19,6 +19,7 @@
 - [Seller pays hold invoice](./seller_pay_hold_invoice.md)
 - [Fiat sent](./fiatsent.md)
 - [Release](./release.md)
+- [Payment Failed](./payment_failed.md)
 - [Rate user](./user_rating.md)
 - [Cancel](./cancel.md)
 - [Peer-to-peer Chat](./chat.md)

src/payment_failed.md

@@ -0,0 +1,112 @@
+# Payment Failed
+
+After the seller releases the sats, Mostro attempts to pay the buyer's Lightning invoice. If this payment fails, Mostro sends a `payment-failed` action to the buyer.
+
+> ⚠️ **Important:** `payment-failed` is an **Action** (notification), NOT an order **Status**. The order remains in `settled-hold-invoice` status while Mostro retries the payment.
+
+## When Payment Fails
+
+When Mostro cannot pay the buyer's Lightning invoice, it sends this message to the buyer:
+
+```json
+[
+  {
+    "order": {
+      "version": 1,
+      "id": "<Order Id>",
+      "action": "payment-failed",
+      "payload": {
+        "payment_failed": {
+          "payment_attempts": 3,
+          "payment_retries_interval": 5
+        }
+      }
+    }
+  },
+  null
+]
+```
+
+### Payload Fields
+
+| Field | Type | Description |
+|-------|------|-------------|
+| `payment_attempts` | integer | Number of retry attempts remaining |
+| `payment_retries_interval` | integer | Minutes between retry attempts |
+
+## What Happens Next
+
+1. **Automatic retries**: Mostro will automatically retry the payment according to `payment_attempts` and `payment_retries_interval`
+
+2. **Order status unchanged**: The order remains in `settled-hold-invoice` status during retries
+
+3. **All retries failed**: If all payment attempts fail, Mostro sends `add-invoice` action requesting the buyer to provide a new Lightning invoice:
+
+```json
+[
+  {
+    "order": {
+      "version": 1,
+      "id": "<Order Id>",
+      "action": "add-invoice",
+      "payload": {
+        "order": {
+          "id": "<Order Id>",
+          "amount": 50000,
+          ...
+        }
+      }
+    }
+  },
+  null
+]
+```
+
+4. **Buyer provides new invoice**: The buyer must submit a new Lightning invoice for the sats to be delivered
+
+## Client Implementation Notes
+
+- **Do NOT create a "PaymentFailed" order status** — this action is a notification only
+- Display a message to the buyer explaining the payment failed and retries are in progress
+- When `add-invoice` is received after failed retries, prompt the buyer to provide a new invoice
+- The sats remain safely locked in escrow (`settled-hold-invoice`) throughout this process
+
+## Flow Diagram
+
+```text
+Seller releases sats
+        │
+        ▼
+Order status: settled-hold-invoice
+        │
+        ▼
+Mostro attempts Lightning payment
+        │
+        ├─── Success ──────────────► Order status: success
+        │                            (buyer receives purchase-completed)
+        │
+        └─── Failure
+              │
+              ▼
+        Action: payment-failed (to buyer)
+        Order status: still settled-hold-invoice
+              │
+              ▼
+        Mostro retries (up to payment_attempts times)
+              │
+              ├─── Retry succeeds ──► Order status: success
+              │
+              └─── All retries fail
+                    │
+                    ▼
+              Action: add-invoice (to buyer)
+              Buyer provides new invoice
+                    │
+                    ▼
+              Mostro pays new invoice ──► Order status: success
+```
+
+## Related Actions
+
+- [release](./release.md) - Seller releases sats
+- [add-invoice](./take_sell.md) - Buyer provides Lightning invoice

src/release.md

@@ -54,7 +54,9 @@ And a message to the buyer to let him know that the sats were released:
 
 ## Buyer receives sats
 
-Right after seller release sats Mostro will attempt to pay the buyer's lightning invoice. When the payment succeeds, Mostro will send a message to the buyer indicating that the purchase was completed:
+Right after seller releases sats, Mostro will attempt to pay the buyer's Lightning invoice. When the payment succeeds, Mostro will send a message to the buyer indicating that the purchase was completed:
+
+> **Note:** If the Lightning payment fails, Mostro will retry automatically. See [Payment Failed](./payment_failed.md) for details on how payment failures are handled.
 
 ```json
 [