Switching Shopkeeper Prices by Quantity with `OnSHKcalcTotalPrice`

Quantity-based pricing is one of the most common ecommerce requirements, and Shopkeeper can handle it cleanly when the recalculation event updates the purchase data before totals are finalized.

The original solution listened to OnSHKcalcTotalPrice and switched between two TVs, price and price2, depending on how many units were in the cart:

if ($e->name == 'OnSHKcalcTotalPrice') {
    if (isset($_SESSION['purchases'])) {
        $purchases = unserialize($_SESSION['purchases']);
        foreach ($purchases as $kk => $val) {
            if ($val[1] < 11) $sk = 'price';
            if ($val[1] > 10) $sk = 'price2';
            $tv = $modx->getTemplateVarOutput(array($sk), $val[0]);
            $purchases[$kk][2] = $tv[$sk];
        }
        $_SESSION['purchases'] = serialize($purchases);
        $e->params['purchases'] = $purchases;
    }
}

The key line is the last one. Updating $e->params['purchases'] makes sure Shopkeeper continues its calculation using the modified prices instead of stale session data.

This is a good example of a lightweight quantity-tier system that stays inside the existing Evolution CMS + Shopkeeper workflow instead of requiring a separate pricing engine.