Adjusters are classes that return adjustment models to the cart. Adjustment models contain an amount which modifies the price of the order. Adjustment models always belong to the order, but can optionally belong to a line item.
An Adjuster class implements the Adjuster Interface found at vendor/craftcms/commerce/src/base/AdjusterInterface.php.
Simply append your adjuster class to the types array in the OrderAdjustments::EVENT_REGISTER_ORDER_ADJUSTERS event model.
Event::on(OrderAdjustments::class, OrderAdjustments::EVENT_REGISTER_ORDER_ADJUSTERS, function(RegisterComponentTypesEvent $event) {
$event->types[] = MyAdjuster::class;
});The order of the adjustments within the types array is important, as it is the order they will be run through on the order.
You could have a project level event listener, that could reorder these adjusters, and not append any new ones.
Each order adjustment model should contain all information about how the adjuster came to it's adjustment. For example, the shipping adjuster includes the information about the matching shipping rules used to calculate the shipping cost, and stores the rule information in the sourceSnapshot attribute of the adjustment model.
The amount value on the Order Adjustment Model is used when totalling the cart. Use negative amounts to reduce the price of the order.
If you need to explain in plain text the adjustment made use the description field on the Model.
If you mark the adjustment model's included attribute as true, the adjustment does not make any changes to the orders total, but simply records an amount that was included in the price of the order.
The only included adjustment we use in the core commerce at the moment is included taxes.