[8.x] Document invoice renderer (laravel#7639)

* Update billing.md

* Update billing.md

* Update billing.md

Co-authored-by: Taylor Otwell <[email protected]>

Author: driesvints

billing.md

@@ -1586,6 +1586,35 @@ The `downloadInvoice` method also allows for a custom filename via its third arg
 
     return $request->user()->downloadInvoice($invoiceId, [], 'my-invoice');
 
+<a name="custom-invoice-render"></a>
+#### Custom Invoice Renderer
+
+Cashier also makes it possible to use a custom invoice renderer. By default, Cashier uses the `DompdfInvoiceRenderer` implementation, which utilizes the [dompdf](https://github.com/dompdf/dompdf) PHP library to generate Cashier's invoices. However, you may use any renderer you wish by implementing the `Laravel\Cashier\Contracts\InvoiceRenderer` interface. For example, you may wish to render an invoice PDF using an API call to a third-party PDF rendering service:
+
+    use Illuminate\Support\Facades\Http;
+    use Laravel\Cashier\Contracts\InvoiceRenderer;
+    use Laravel\Cashier\Invoice;
+
+    class ApiInvoiceRenderer implements InvoiceRenderer
+    {
+        /**
+         * Render the given invoice and return the raw PDF bytes.
+         *
+         * @param  \Laravel\Cashier\Invoice. $invoice
+         * @param  array  $data
+         * @param  array  $options
+         * @return string
+         */
+        public function render(Invoice $invoice, array $data = [], array $options = []): string
+        {
+            $html = $invoice->view($data)->render();
+
+            return Http::get('https://example.com/html-to-pdf', ['html' => $html])->get()->body();
+        }
+    }
+
+Once you have implemented the invoice renderer contract, you should update the `cashier.invoices.renderer` configuration value in your application's `config/cashier.php` configuration file. This configuration value should be set to the class name of your custom renderer implementation.
+
 <a name="checkout"></a>
 ## Checkout