diff --git a/public/pos/pos-app.js b/public/pos/pos-app.js
index 0f1f097..156e0d9 100644
--- a/public/pos/pos-app.js
+++ b/public/pos/pos-app.js
@@ -783,6 +783,26 @@ function clearCust(){
 }
 
 // ---------------- save sale ----------------
+// Minimal, self-contained error toast. `toast.error(...)` below used
+// to reference a `toast` global that was never defined anywhere in
+// this app (no library included, no local implementation) — every
+// checkout rejection (e.g. insufficient stock) threw a
+// ReferenceError instead of showing the operator anything, so the
+// cart just sat there with no feedback.
+function showToast(message){
+  let el = document.getElementById('posToast');
+  if(!el){
+    el = document.createElement('div');
+    el.id = 'posToast';
+    el.style.cssText = 'position:fixed;left:16px;right:16px;bottom:calc(96px + env(safe-area-inset-bottom));max-width:448px;margin:0 auto;background:#c92a2a;color:#fff;padding:14px 16px;border-radius:12px;font-size:14px;font-weight:600;line-height:1.4;box-shadow:0 8px 24px rgba(0,0,0,.25);z-index:999;opacity:0;transition:opacity .15s ease';
+    document.body.appendChild(el);
+  }
+  el.textContent = message;
+  el.style.opacity = '1';
+  clearTimeout(el._hideTimer);
+  el._hideTimer = setTimeout(()=>{ el.style.opacity = '0'; }, 5000);
+}
+
 async function confirmSale(){
   const btn = document.getElementById('confirmBtn');
   if(btn.disabled) return;
@@ -905,9 +925,9 @@ async function confirmSale(){
           : s.stock_item_id;
         return `${name}: only ${s.available} in stock (you asked for ${s.requested})`;
       });
-      toast.error('Not enough stock — ' + lines.join('; '));
+      showToast('Not enough stock — ' + lines.join('; '));
     } else {
-      toast.error('Sale rejected: ' + (serverError.kind === 'other' ? serverError.message : 'Unknown error'));
+      showToast('Sale rejected: ' + (serverError.kind === 'other' ? serverError.message : 'Unknown error'));
     }
     if (btn) {
       btn.disabled = false;
diff --git a/public/pos/sw.js b/public/pos/sw.js
index 8249748..1e53a11 100644
--- a/public/pos/sw.js
+++ b/public/pos/sw.js
@@ -1,5 +1,5 @@
 // Service worker — cache app shell, network-first for HTML + theme.js, cache-first for other static.
-const CACHE = "pos-v20";
+const CACHE = "pos-v21";
 const APP_SHELL = [
   './',
   './index.html',