diff --git a/nostr_publisher.py b/nostr_publisher.py
index a6d487b..240b406 100644
--- a/nostr_publisher.py
+++ b/nostr_publisher.py
@@ -39,12 +39,23 @@ def build_nip52_event(event: Event, pubkey: str) -> NostrEvent:
 
     Time-based (kind 31923) if event_start_date carries an HH:MM, otherwise
     date-based (kind 31922). Tags:
-      d       - event.id
-      title   - event.name
-      start   - unix timestamp (31923) or YYYY-MM-DD (31922)
-      end     - same encoding (optional)
+      d                  - event.id
+      title              - event.name
+      start              - unix timestamp (31923) or YYYY-MM-DD (31922)
+      end                - same encoding (optional)
       image, location, t (categories) - optional
+      tickets_available  - current remaining capacity (omitted when unlimited)
+      tickets_sold       - running paid-count (always emitted; clients can
+                           derive original_capacity = available + sold)
+      tickets_price      - price_per_ticket (always emitted; 0 means free)
+      tickets_currency   - the currency string
     Content:  event.info
+
+    The four ticket_* tags are AIO custom additions outside the NIP-52
+    spec; spec-compliant clients ignore unknown tags so this stays
+    backwards-compatible. They let connected clients render the
+    "X tickets remaining" badge and the Buy CTA without an extra REST hop,
+    and pick up live inventory updates via the same relay subscription.
     """
     time_based = _has_time(event.event_start_date)
     kind = 31923 if time_based else 31922
@@ -81,6 +92,15 @@ def build_nip52_event(event: Event, pubkey: str) -> NostrEvent:
     for cat in event.categories or []:
         tags.append(["t", cat])
 
+    # `amount_tickets == 0` means unlimited capacity in this extension's
+    # schema. Omitting the tag is how clients distinguish unlimited from
+    # "0 left" (sold out).
+    if event.amount_tickets > 0:
+        tags.append(["tickets_available", str(event.amount_tickets)])
+    tags.append(["tickets_sold", str(event.sold)])
+    tags.append(["tickets_price", str(event.price_per_ticket)])
+    tags.append(["tickets_currency", event.currency])
+
     nostr_event = NostrEvent(
         pubkey=pubkey,
         created_at=int(time.time()),
diff --git a/services.py b/services.py
index 159bbdc..0a2de28 100644
--- a/services.py
+++ b/services.py
@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import asyncio
 from asyncio.tasks import create_task
 
 from lnbits.core.models.users import UserNotifications
@@ -21,6 +22,7 @@ from .crud import (
     update_ticket,
 )
 from .models import Event, Ticket
+from .nostr_hooks import publish_or_delete_nostr_event
 
 DEFAULT_NOSTR_RELAYS = [
     "wss://relay.damus.io",
@@ -28,19 +30,42 @@ DEFAULT_NOSTR_RELAYS = [
     "wss://relay.nostr.band",
 ]
 
+# Per-event lock: serializes the counter-update + Nostr republish for a
+# single event_id so two paid invoices landing on the listener queue back-
+# to-back can't reorder the published state. Lazy-populated; entries are
+# left in memory for the lifetime of the process (cheap — one asyncio.Lock
+# object per event ever sold).
+_event_paid_locks: dict[str, asyncio.Lock] = {}
+
+
+def _event_paid_lock(event_id: str) -> asyncio.Lock:
+    lock = _event_paid_locks.get(event_id)
+    if lock is None:
+        lock = asyncio.Lock()
+        _event_paid_locks[event_id] = lock
+    return lock
+
 
 async def set_ticket_paid(ticket: Ticket) -> Ticket:
     if ticket.paid:
         return ticket
 
-    ticket.paid = True
-    await update_ticket(ticket)
+    async with _event_paid_lock(ticket.event):
+        ticket.paid = True
+        await update_ticket(ticket)
 
-    event = await get_event(ticket.event)
-    assert event, "Couldn't get event from ticket being paid"
-    event.sold += 1
-    event.amount_tickets -= 1
-    await update_event(event)
+        event = await get_event(ticket.event)
+        assert event, "Couldn't get event from ticket being paid"
+        event.sold += 1
+        event.amount_tickets -= 1
+        await update_event(event)
+
+        # Republish the NIP-52 calendar event so connected clients see
+        # the new tickets_available / tickets_sold counters via their
+        # existing relay subscription. Failures are logged + swallowed
+        # inside publish_or_delete_nostr_event so a Nostr outage doesn't
+        # break the payment flow.
+        await publish_or_delete_nostr_event(event)
 
     return ticket