Building Quantity-Based Cart Discounts with OnSHKcalcTotalPrice

Shopkeeper gives you a useful extension point in OnSHKcalcTotalPrice, but the first examples teams find often solve only the simplest case: discount a single product when its quantity crosses a threshold. the original question pushed the logic further by asking for progressive discounts based on how many items are in the cart overall.

That is a better fit for custom total-price calculation, because the discount rule is about the cart state, not a single line item.

Base pattern

$e = &$modx->Event;
$output = 0;

if ($e->name == 'OnSHKcalcTotalPrice') {
    $discountMap = [2 => 0.05, 3 => 0.10, 4 => 0.15];
    $itemsCount = 0;

    foreach ($purchases as $item) {
        $itemsCount += (int) $item[1];
    }

    $discount = 0;
    foreach ($discountMap as $threshold => $value) {
        if ($itemsCount >= $threshold) {
            $discount = $value;
        }
    }

    foreach ($purchases as $item) {
        $price = round($item[2] * (1 - $discount), 2);
        $output += $item[1] * $price;
    }

    $e->output($output);
}

Why this version is different

The common forum example checks whether one product quantity is greater than a threshold. This version first calculates the total number of units in the cart and then applies the correct discount tier to the full order logic.

That makes it much better for promotions like “5% off from the second item, 10% from the third, 15% from the fourth.”