// config/subscribd.php
'gateways' => [
    'fastspring' => [
        'driver'         => \Pixelworxio\Subscribd\Gateways\FastSpringGateway::class,
        'username'       => env('FASTSPRING_USERNAME'),
        'password'       => env('FASTSPRING_PASSWORD'),
        'webhook_secret' => env('FASTSPRING_WEBHOOK_SECRET'),
        'storefront'     => env('FASTSPRING_STOREFRONT_URL'),
        'success_url'    => env('FASTSPRING_SUCCESS_URL'),
        'cancel_url'     => env('FASTSPRING_CANCEL_URL'),
    ],
],

FASTSPRING_USERNAME=...
FASTSPRING_PASSWORD=...
FASTSPRING_WEBHOOK_SECRET=...
FASTSPRING_STOREFRONT_URL=https://yourstore.onfastspring.com
FASTSPRING_SUCCESS_URL=https://yourapp.com/billing/success
FASTSPRING_CANCEL_URL=https://yourapp.com/billing/cancel

$subscription = app(CreateSubscription::class)->execute($user, $plan, ['gateway' => 'fastspring']);

if ($subscription->status->value === 'incomplete') {
    return redirect($subscription->meta['checkout_url']);
}