This question is about the "correct" design pattern, not about functional code. I want to adhere to best-practices and use the right feature in Laravel.
I have a model called Order which contains users' product orders.
Order has several columns, like which product, quantity, etc, and is stored in mysql, with a belongsTo() call to the User model.
When I place an order using the OrderController, I call an outside API that I set up using a Service class.
Here's the main part of the question:
I need to add certain fields that the API requires, but on my end are always the same, so it would make sense to me to pack these into an object of their own and just append that object to the end of my Order data before submission.
So where is the "Best" place to put this extra data? In my model? In a Service class? I'm leaning toward the service class, but that just doesn't feel right.
You have an action that gives a single or a collection of a model. So the best practice for adding some extra data to those results is using JsonResource and ResourceCollection. By using them you can easily add anything you want in the ToArray method.
Lumen doesn't have Illuminate\Http by default but you can add it to your project.
Official Http package of laravel
Eloquent: API Resources Documentation.