Replacing Checkout (Client-Side)

Use this approach when you want to redirect shoppers to Simpler Checkout directly from your frontend — for example, when they click "Proceed to Checkout" in your cart page. No backend code is required.

If you need server-side control over the cart data before redirecting, see Replacing Checkout (Server-Side) instead.

No Authentication Required

This redirect does not require any authentication headers or secrets. You only need your App ID.

Overview

The flow has two steps:

1. Build a checkout URL containing the shopper's cart data as query parameters.
2. Redirect the shopper's browser to that URL.

After the redirect, the shopper lands on the Simpler Checkout page where they complete their purchase. Simpler will then call your Platform Interface APIs to price the cart, resolve shipping, and submit the order.

URL Structure

https://checkout.simpler.so/?app=<APP_ID>&prd=<ENCODED_PRODUCT_DATA>&cur=<CURRENCY>&lang=<LANGUAGE>

URL Parameters

Parameter	Type	Required	Description
app	string	Yes	Your Simpler Store App ID.
prd	string	Yes	URL-encoded JSON array of cart items. See below.
cur	string	Yes	ISO 4217 currency code (EUR, USD, GBP).
lang	string	Yes	IETF language tag per RFC 5646 (e.g. en, el, fr).

prd Item Schema

Each item in the prd JSON array has the following fields:

Field	Type	Required	Description
id	string	Yes	Product identifier as used in your Platform Interface product catalog.
qty	integer	Yes	Number of units.
attrs	object	No	Key-value pairs that identify a specific product variant (e.g. {"color": "blue", "size": "L"}).

PHP Example

function build_checkout_url($cart, $app_id, $currency_iso, $language_iso, $endpoint = 'https://checkout.simpler.so/') {
    if (!$cart || empty($cart['items'])) {
        error_log('Cart is empty or not available.');
        return '';
    }

    $products_array = [];

    foreach ($cart['items'] as $item) {
        $product_data = [
            'id'  => (string)$item['id'],
            'qty' => (int)$item['quantity'],
        ];

        if (!empty($item['attributes'])) {
            $product_data['attrs'] = (object)$item['attributes'];
        }

        $products_array[] = $product_data;
    }

    $products_string = urlencode(json_encode($products_array));

    return $endpoint .
        '?app=' . $app_id .
        '&prd=' . $products_string .
        '&cur=' . $currency_iso .
        '&lang=' . $language_iso;
}

Usage

$cart = [
    'items' => [
        [
            'id' => 123,
            'quantity' => 1,
            'attributes' => [
                'color' => 'blue',
                'size' => 'L'
            ]
        ],
        [
            'id' => 456,
            'quantity' => 2,
            'attributes' => []
        ]
    ]
];

$app_id = '550e8400-e29b-41d4-a716-446655440000';
$checkout_url = build_checkout_url($cart, $app_id, 'EUR', 'el');

header("Location: $checkout_url");
exit;