diff --git a/README.md b/README.md
index 2f3e66a..8c3f8e1 100644
--- a/README.md
+++ b/README.md
@@ -1,2 +1,32 @@
-# vinted-favs-price
+# Vinted Favorites Price Calculator
+A simple browser extension that calculates and displays the total price of your favorite items on [vinted.de](https://www.vinted.de/member/items/favourite_list). You can toggle between the base price and the price including Vinted's buyer protection.
 
+## Features
+
+- Adds a floating button to your favorites list page.
+- Calculates the total sum of all visible favorite items.
+- Toggle between base price and price including buyer protection.
+- Updates automatically as you scroll and more items load.
+
+## Installation
+
+### Download
+
+- **[Get it for Firefox on Mozilla Add-ons](https://addons.mozilla.org/en-US/firefox/addon/vinted-favorites-price/)**
+
+Click the link above to download and install the extension directly from the official Firefox Add-ons store.
+
+## Usage
+
+1. Go to [https://www.vinted.de/member/items/favourite_list](https://www.vinted.de/member/items/favourite_list).
+2. Wait for the page to load. A button labeled "Show Total Price (Incl. Protection)" will appear in the bottom right corner.
+3. Click the button to toggle between showing the base price and the price including buyer protection.
+4. The total sum will be displayed in the page header.
+
+## Notes
+
+- The sum updates when you refresh the page or scroll to load more items.
+- The extension is designed to work on the favorites list page and may not function correctly on other pages of the Vinted website.
+
+## License
+[MIT License](LICENSE)
diff --git a/calculate-total.js b/calculate-total.js
new file mode 100644
index 0000000..3d10d90
--- /dev/null
+++ b/calculate-total.js
@@ -0,0 +1,89 @@
+// Configuration & State
+let showProtection = false;
+const GRID_SELECTOR = '.feed-grid .feed-grid__item';
+const H1_SELECTOR = 'h1.web_ui__Text__text.web_ui__Text__heading.web_ui__Text__left.web_ui__Text__parent';
+
+// 1. Create and Inject the Toggle Button
+const btn = document.createElement('button');
+btn.id = 'vinted-helper-toggle';
+btn.innerText = "Show Total Price (Incl. Protection)";
+btn.style.cssText = `
+    position: fixed;
+    bottom: 20px;
+    right: 20px;
+    z-index: 10000;
+    padding: 12px 20px;
+    background: #007782;
+    color: white;
+    border: 2px solid #005f68;
+    border-radius: 8px;
+    cursor: pointer;
+    font-family: sans-serif;
+    font-weight: bold;
+`;
+document.body.appendChild(btn);
+
+// 2. Calculation Logic
+const updateDisplayAndSum = () => {
+    const items = document.querySelectorAll(GRID_SELECTOR);
+    let totalSum = 0;
+    let count = 0;
+
+    items.forEach(item => {
+        const basePriceEl = item.querySelector('[data-testid*="--price-text"]');
+        const protectPriceEl = item.querySelector('[data-testid*="--breakdown"] .web_ui__Text__subtitle');
+
+        if (basePriceEl && protectPriceEl) {
+            // Determine which price to use for the sum
+            const targetEl = showProtection ? protectPriceEl : basePriceEl;
+            const price = parseFloat(targetEl.innerText.replace(/[^\d,.]/g, '').replace(',', '.'));
+
+            if (!isNaN(price)) {
+                totalSum += price;
+                count++;
+            }
+
+            // Visual Toggling
+            if (showProtection) {
+                basePriceEl.style.display = 'none';
+                protectPriceEl.style.color = '#eb5a62';
+                protectPriceEl.style.fontWeight = 'bold';
+                protectPriceEl.style.fontSize = '1.1em';
+            } else {
+                basePriceEl.style.display = 'block';
+                protectPriceEl.style.color = '';
+                protectPriceEl.style.fontWeight = '';
+                protectPriceEl.style.fontSize = '';
+            }
+        }
+    });
+
+    // Update the H1 Header
+    const header = document.querySelector(H1_SELECTOR);
+    if (header && count > 0) {
+        const formattedSum = totalSum.toLocaleString('de-DE', { minimumFractionDigits: 2, maximumFractionDigits: 2 });
+        const label = showProtection ? "Total (with Prot.)" : "Total";
+        
+        // Remove old brackets if they exist, then append new total
+        const cleanTitle = header.innerText.split(' [')[0];
+        header.innerText = `${cleanTitle} [${label}: €${formattedSum}]`;
+    }
+};
+
+// 3. Event Listener
+btn.addEventListener('click', () => {
+    showProtection = !showProtection;
+    btn.innerText = showProtection ? "Show Base Price" : "Show Total Price (Incl. Protection)";
+    btn.style.background = showProtection ? "#eb5a62" : "#007782";
+    updateDisplayAndSum();
+});
+
+// Initial run
+setTimeout(updateDisplayAndSum, 2000);
+
+// Update sum when scrolling (lazy loading support)
+let scrollTimeout;
+window.addEventListener('scroll', () => {
+    clearTimeout(scrollTimeout);
+    scrollTimeout = setTimeout(updateDisplayAndSum, 500);
+});
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diff --git a/manifest.json b/manifest.json
new file mode 100644
index 0000000..e5cf49b
--- /dev/null
+++ b/manifest.json
@@ -0,0 +1,18 @@
+{
+  "manifest_version": 2,
+  "name": "vinted-favs-price",
+  "version": "1.0",
+  "content_scripts": [
+    {
+      "matches": ["https://www.vinted.de/member/items/favourite_list"],
+      "js": ["calculate-total.js"]
+    }
+  ],
+  "browser_specific_settings": {
+    "gecko": {
+      "data_collection_permissions": {
+        "required": ["websiteContent"]
+      }
+    }
+  }
+}
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