> ## Documentation Index
> Fetch the complete documentation index at: https://docs.lunarphp.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Extending Taxation

Taxation is driver-based, allowing custom tax calculation logic to replace or extend the default system.

## Overview

Taxation is complex and sometimes what Lunar offers out of the box is not sufficient. This is why taxation is driver-based, allowing custom logic to be added as needed.

By default, a `SystemTaxDriver` is used, which relies on Lunar's internal models and database as outlined in the [Taxation](/1.x/reference/taxation) reference. To use a custom implementation, the driver can be changed in the `config/lunar/taxes.php` config file.

```php theme={null}
<?php

return [
    'driver' => 'system',
];
```

## Writing a Custom Driver

A custom driver must implement the `Lunar\Base\TaxDriver` interface:

```php theme={null}
<?php

namespace App\Drivers;

use Lunar\Base\Addressable;
use Lunar\Base\Purchasable;
use Lunar\Base\TaxDriver;
use Lunar\Base\ValueObjects\Cart\TaxBreakdown;
use Lunar\Models\Contracts\CartLine;
use Lunar\Models\Contracts\Currency;

class TaxJar implements TaxDriver
{
    /**
     * Set the shipping address.
     */
    public function setShippingAddress(?Addressable $address = null): self
    {
        // ...
        return $this;
    }

    /**
     * Set the billing address.
     */
    public function setBillingAddress(?Addressable $address = null): self
    {
        // ...
        return $this;
    }

    /**
     * Set the currency.
     */
    public function setCurrency(Currency $currency): self
    {
        // ...
        return $this;
    }

    /**
     * Set the purchasable item.
     */
    public function setPurchasable(Purchasable $purchasable): self
    {
        // ...
        return $this;
    }

    /**
     * Set the cart line.
     */
    public function setCartLine(CartLine $cartLine): self
    {
        // ...
        return $this;
    }

    /**
     * Return the tax breakdown from a given sub total.
     *
     * @param  int  $subTotal
     */
    public function getBreakdown($subTotal): TaxBreakdown
    {
        // Build and return a TaxBreakdown instance...
    }
}
```

Once the driver is created, register it by extending the tax manager in a service provider:

```php theme={null}
public function boot()
{
    \Lunar\Facades\Taxes::extend('taxjar', function ($app) {
        return $app->make(TaxJar::class);
    });
}
```

Then set the driver in the `config/lunar/taxes.php` config:

```php theme={null}
<?php

return [
    'driver' => 'taxjar',
];
```