Laravel 8 Unit Testing - assertJson() how to capture it in controller

I have a laravel 8 Unit Test which looks like this:

public function testAddingTwoCars()
{
    $response = $this->postJson('api/basket', ['cars' => [['name' => 'car one'], ['name' => 'car two']]]);
    $response
        ->assertStatus(200)
        ->assertJson(['total' => 40]);
}

In my route I have:

Route::post('/api/basket',[basketController::class, 'store']);

In my controller I have:

public function store(Request $request)
{
    $data = $request->all();
    return response()->json($data);
}

when I run the test with php artisan test it shows this:

 • Tests\Unit\BasketTest > adding two cars
 Unable to find JSON:                                        
                                                                                                                                                    
 [{                                                          
     "total": 40                                             
 }]                                                          
                                                                                                                                                                      
 within response JSON:                                       
                                                        
 [{                                                          
   "cars": [                                           
     {                                                   
         "name": "car one"                                 
     },                                                  
     {                                                   
         "name": "car two"                                 
     }                                                   
   ]                                                       
 }].      

How do I:grab the total as its not in $request so that I could do something like this in my controller:

$public function store(Request $request)
{
    $data = $request->all();
    $data['total'] = 40;
    return response()->json($data);
}  

The above works, but it is hard coded so is not the best way to do it