Retrieving/calculating Order total from OrderItems

I'm new to Laravel and trying to understand best practices as I build an eCommerce system.

Note, I'm on L5.1 atm, moving to 5.7, so solution must work on 5.1 right now.

I have an Order:

class Order extends Model
{
...
    public function items()
    {
        return $this->hasMany('Statamic\Addons\Buck\Models\OrderItem');
    }
}

and an OrderItem:

class OrderItem extends Model
{
...
    public function order()
    {
        return $this->belongsTo('Statamic\Addons\Buck\Models\Order');
    }

    public function getTotalAttribute()
    {
        return $this->quantity * $this->price;
    }
}

To get the Order total, I know I can use the collection sum method, but is that the "right" way to do it? Or is there an Eloquent way to do this that's better or more standard?

Thanks in advance for your help.