Shopping Cart Total Tax Estimator

WHAT IT DOES

This script estimates the total cost of your shopping cart including tax, even when the site doesn't show it upfront. When you're on a cart or checkout page, the script detects the subtotal, shipping costs, and any displayed tax amounts. If no tax is shown, it calculates an estimate based on your configured tax rate. The panel displays a clear breakdown: subtotal, shipping, tax (with an indicator of whether it's actual or estimated), and the estimated total. You can set a global tax rate or per-site rates for different regions or stores. The estimate updates dynamically as you change quantities or add/remove items.

WHERE IT RUNS

The script runs on all websites (*://*/*), making it truly universal. It works on cart and checkout pages from Amazon, Walmart, Target, Best Buy, and any other e-commerce site. It supports both desktop and mobile views.

HOW TO USE

After installation, the settings panel appears on the left side of the screen with the gear emoji (⚙️). Open the panel to configure the script:

- The "Enable tax estimator" toggle enables or disables the script.

- Set a "Global tax rate" (percentage) for all sites.

- Set a "Per-site tax rate" to override the global rate for a specific store.

- Click "Refresh estimate" to manually update the calculation.

- Click "Clear per-site rates" to remove all custom rates.

The estimate appears in the panel whenever you're on a cart or checkout page. If the page already shows tax, the script displays the actual amount.

TECHNICAL SECTION — REAL MECHANISM

The script detects cart and checkout pages by scanning for common e-commerce elements like .subtotal, .cart-subtotal, .order-total, .shipping, .tax, and .grand-total. It extracts numeric values from these elements using regex to find amounts like "$19.99" or "19.99". The subtotal is typically the sum of products before tax and shipping. The script estimates tax by multiplying the subtotal by the configured tax rate (e.g., 8.5% becomes 0.085). If the page already displays a tax amount, the script uses that instead of estimating. The estimated total is calculated as subtotal + shipping + tax, rounded to the nearest cent. Per-site tax rates are stored as an object keyed by the site name (e.g., "amazon", "walmart") via GM_setValue, with the global rate as a fallback. The display is updated every few seconds using a MutationObserver to watch for cart changes.

THE PANEL

The settings panel is built with BotGentz's BGPanel component. It is draggable via its header, allowing you to reposition it anywhere on the screen. The panel snaps to the left edge by default but can be moved freely. Click the gear emoji to collapse the panel into a small bubble, and click the bubble to expand it again. The panel remembers its position per page session. Press the Escape key to quickly close or minimize the panel. The panel includes a status flash bar that provides visual feedback when settings are changed or actions are performed. All settings save automatically via GM_setValue.

PLEASE NOTE

This script requires the free BotGentz browser extension to run. It does not function as a standalone Chrome extension and does not use chrome.* APIs. The script estimates tax based on the user's configured rate—it does not automatically detect the correct tax rate for your location. You must set the tax rate manually. Some e-commerce sites may show tax at the final checkout step or after entering shipping details; the script will show the actual tax once it appears on the page. The script never reads or writes password fields, hidden inputs, or file inputs. All DOM operations use safe methods (style properties, textContent, appendChild) and never use innerHTML.