Custom Contao Isotope Surcharge

config.php

<?php

/**
 * Register Isotope model types
 */
\Isotope\Model\ProductCollectionSurcharge::registerModelType('mysurcharge', \MySurchargeModel::class);

/**
 * Register Isotope hooks
 */
$GLOBALS['ISO_HOOKS']['findSurchargesForCollection'][] = [\MySurcharge::class, 'findSurchargesForCollection'];

MySurcharge.php

<?php

use Isotope\Model\ProductCollection;

class MySurcharge
{
    public function findSurchargesForCollection(ProductCollection $collection): array
    {
        return [\MySurchargeModel::build()];
    }
}

MySurchargeModel.php

<?php

use Isotope\Interfaces\IsotopeProductCollectionSurcharge;
use Isotope\Model\ProductCollectionSurcharge;

class MySurchargeModel extends ProductCollectionSurcharge implements IsotopeProductCollectionSurcharge
{
    public static function build(): self
    {
        $surcharge = new self();
        $surcharge->label = 'My Surcharge';
        $surcharge->price = '';
        $surcharge->total_price = 100.0;
        $surcharge->tax_class = 1;
        $surcharge->before_tax = true;
        $surcharge->addToTotal = true;

        return $surcharge;
    }
}