An order total extension

The sum of an order is assembled from lines: products subtotal, shipping, coupon, tax, total. Each line is a separate extension of the type total. To add a fee or a discount of your own, you write one more such extension — here it is a fee for small orders.

Files

A total consists of a settings page in the admin panel and a model on the storefront:

extension/acme_hello/
├── admin/
│   ├── controller/total/small_order.php
│   ├── language/en-gb/total/small_order.php
│   └── view/template/total/small_order.twig
└── catalog/
    ├── model/total/small_order.php
    └── language/en-gb/total/small_order.php

The admin part is written exactly like the settings page of the module, only the namespace ends with Total, the route is extension/acme_hello/total/small_order and the code of the settings is total_small_order. Two fields are needed: total_small_order_status and total_small_order_sort_order.

The model

// extension/acme_hello/catalog/model/total/small_order.php
namespace Opencart\Catalog\Model\Extension\AcmeHello\Total;
 
class SmallOrder extends \Opencart\System\Engine\Model
{
    public function getTotal(array &$totals, array &$taxes, float &$total): void
    {
        if ($this->cart->getSubTotal() >= 20) {
            return;
        }
 
        $this->load->language('extension/acme_hello/total/small_order');
 
        $fee = 3.0;
 
        $totals[] = [
            'extension' => 'acme_hello',
            'code' => 'small_order',
            'title' => $this->language->get('text_small_order'),
            'value' => $fee,
            'sort_order' => (int) $this->config->get('total_small_order_sort_order'),
        ];
 
        $total += $fee;
    }
}

The core calls getTotal() of every enabled total in turn. The method receives three values by reference: the list of lines, the taxes and the running sum. It adds its line and increases the sum.

// extension/acme_hello/catalog/language/en-gb/total/small_order.php
$_['text_small_order'] = 'Small order fee';

Turning it on

1. Open Extensions → Extensions and choose the type Order Totals.
2. Install «Small order fee», open its settings, turn the status on and set the sort order.

Put a product cheaper than $20 into the cart and open index.php?route=checkout/cart — under the subtotal there is the line «Small order fee $3.00», and the total is larger by this sum.

If the new item is missing from the list, rebuild the archive and install the package again in Extensions → Installer: the list of files of a package is recorded during installation.

Sort order

Lines are calculated in the order of the sort_order setting, so its value matters:

A fee with the order 2 is calculated after the products and before the tax. See the real values of your shop in the settings of each total.

A discount

A discount is the same line with a negative value: 'value' => -5.0 and $total -= 5.0.

Common mistake

The line is added to $totals, but $total is not changed. The cart shows the fee, and the final sum stays the same — the customer pays less than the lines add up to. Every line added to the list must also change the running sum by the same value.