How do I extend pipelines?

The payload data can be altered using the 
extend() method. This is typically done from within a Service Provider:
If the code is unique to a particular store it could be placed in the 
boot() method of the 
AppServiceProvider.
If it is to be included as part of a module, then it can be added to the setup method of the module's Service Provider.
For example, if a module needs to add an option to items to flag them as requiring gift wrap, the 
CartItemBuilder pipeline class should be extended in the module's ServiceProvider as shown below:
<?php
namespace Aerocargo\Acme;
use Aero\Cart\CartItemOption;
use Aero\Common\Providers\ModuleServiceProvider;
use Aero\Store\Pipelines\CartItemBuilder;
class ServiceProvider extends ModuleServiceProvider
{
    public function setup() 
    {
        CartItemBuilder::extend(function ($item) {
            if (request()->has('gift_wrap')) {
                $option = CartItemOption::create('Gift wrap');
                $item->addOption($option);
            }
        });
    }
}