Cashier (stripe) subscription increase quantity of optional products

Hello, I'm trying to create a Stripe checkout session for a Subscription with optional multiple products. Some of these products require the user to specify the quantity, but I can't figure out how to tell Stripe to increase the quantity dinamically.

This is what I'm trying to do:

//The $order I'm passing contains a collection with the selected extras
public function createStripeCheckoutSession(Order $order)
{
    //I used this to get the "stripe_price" of the main subscription plan
    $subscription_plan = $order->plans->where('plan_type', 2)->first();
    $checkout = $request->user()->company()->newSubscription('default', $subscription_plan->stripe_price);
    //Here I'm getting the remaining selected products without the $subscrition_plan
    $extras = $order->plans->filter(function($value) {
        return $value->id > 2;
    });
    //Here I tried to loop trough every "extra" and calling the quantity() method of Cashier
    foreach($extras as $extra){
        $checkout->quantity($extra->pivot->quantity, $extra->stripe_price);
    }
      
    $checkout->skipTrial()
    ->allowPromotionCodes()
    ->checkout(
    [
        'success_url' => route('dashboard.subscribed', ['user' => $request->user()]),
        'cancel_url' => route('dashboard')
    ]);

    return $checkout;
}

But instead of returning Stripe's checkout it gives me a

"Symfony\Component\HttpFoundation\Response::setContent(): Argument #1 ($content) must be of type ?string, Laravel\Cashier\SubscriptionBuilder given, called in /var/www/html/vendor/laravel/framework/src/Illuminate/Http/Response.php on line 73"

If I use this instead it returns the stripe checkout, but I don't know how to specify the quantity

return $checkout = $request->user()->company()
        ->newSubscription('default', $subscription_plan->stripe_price)
        ->skipTrial()
        ->allowPromotionCodes()
        ->checkout([
            'success_url' => route('dashboard.subscribed', ['user' => $request->user()]),
            'cancel_url' => route('dashboard'),
        ]);

Thank you very much for your help.