$merx = merx();
$merx->cart()->add(['id' => 'nice-shoes']);

The id has to be a valid page. The page must have a price field.

On another page you should show the customer’s cart.

site/templates/cart.php

$merx = merx();
foreach($merx->cart() as $item) {
  $itemPage = page($item['id']);
  echo $itemPage->title(); // Nice shoes
  echo $itemPage->price(); // 99.99
}

Provide a paymentMethod and other data you need to handle the order (email, name, billing / shipping address etc.).

The payment is validated by the Order Page. Read more about validation.

site/templates/checkout.php

$data = [
  'paymentMethod' => 'paypal',
  'email' => 'chuck@example.com',
  'name' => 'Chuck Norris',
  …
]
try {
  $redirect = merx()->initializePayment($data);
  go($redirect);
} catch (Exception $ex) {
  echo $ex->getMessage();
}

Depending on the payment method the customer is redirected directly or after the customer completed the payment on the payment provider’s external webpage (e.g. PayPal) to the success page.

site/templates/success.php

try {
  $orderPage = merx()->completePayment();
  echo 'Payment completed.';
} catch (Exception $ex) {
  echo $ex->getMessage();
}