Invoking other classes from Acceptance tests in Codeception

I have an acceptance test that requires some setup before it runs. Specifically, I need to start a new subscription in Stripe. I have a class that does this, but when I run the test, Codeception complains that core Laravel classes such as Config cannot be found.

Here's some code, reduced for clarity. First the test:

<?php
// app/tests/browser/stopAndResumeSubscriptionCept.php

use BBapp\providers\payment\stripe\StripeTestBiller;
use BBapp\providers\payment\SubscriptionPlan as Plan;
use BBapp\models\User;

$I = new WebGuy($scenario);
$I->wantTo('stop and then resume my subscription');

// First we need to make a subscription
$biller = new StripeTestBiller;
$user   = User::whereEmail('foo@example.com')->first();
$result = $biller->startSubscription( Plan::monthly(), $user, null );

// Now do acceptance testing stuff
// ...

And here's the StripeTestBiller:

<?php namespace BBapp\providers\payment\stripe;

use BBapp\providers\payment\SubscriptionPlan as Plan;
use BBapp\models\User;

use App;
use Carbon\Carbon;

class StripeTestBiller extends StripeApi {

    public function __construct()
    {
        parent::__construct();
    }

    public function startSubscription( Plan $plan, User $user, $token = null )
    {
        $subscription = Stripe_Customer::create([
            'card' => [
                'number'    => '4242424242424242',
                'exp_month' => '01',
                'exp_year'  => Carbon::now()->addYears(1)->year,
                'cvc'       => '123',
            ],
            'plan'     => $plan->id,
            'email'    => $user->email,
            'metadata' => [
                'user_id'     => $user->id,
                'environment' => App::environment(),
            ],
        ]);

        return $subscription;
    }
    
    // Other methods here
}

Finally, the StripeApi class that StripeTestBiller extends:

<?php namespace BBapp\providers\payment\stripe;

use Config;
use Stripe;

class StripeApi implements StripeInterface {

    public function __construct()
    {
        Stripe::setApiKey( Config::get('stripe.secretKey') );
    }

    // A load of API wrapper methods here
}

The error from the console says:

FATAL ERROR. TESTS NOT FINISHED.
Class 'Config' not found
in *[Path to laravel]*\laravel\app\BBapp\providers\payment\stripe\StripeApi.php:24

...where line 24 is the line in the constructor above.

Is this an autoloading issue? How can I get these classes loaded during acceptance tests?