Amazon Pay Checkout always returns an "InvalidRequest"

Question

It's weird but Amazon Pay always returns an "InvalidRequest" even though the payment configuration was done correctly. Can you see what's wrong here?

FYI, the account has been registered in Europe.

Here's the code repository: https://github.com/heryTz/amazon-pay-poc

Amazon Pay Checkout: documentation

Here is the code:

<?php

require '../vendor/autoload.php';

$GLOBALS['private_key'] = 'path_to_private_key.pem';
$GLOBALS['public_key'] = 'xxx';
$GLOBALS['store_id'] = 'xxx';

function get_client()
{
  $amazonpay_config = array(
    'public_key_id' => $GLOBALS['public_key'],
    'private_key'   => $GLOBALS['private_key'],
    'region'        => 'EU',
    'sandbox'       => true,
    'algorithm' => 'AMZN-PAY-RSASSA-PSS-V2'
  );

  return new Amazon\Pay\API\Client($amazonpay_config);
}

function create_checkout_session_payload()
{
  $payload = json_encode([
    'webCheckoutDetails' => [
      'checkoutReviewReturnUrl' => 'https://a.com/merchant-review-page'
    ],
    'storeId' => $GLOBALS['store_id'],
  ]);

  $client = get_client();
  $signature = $client->generateButtonSignature($payload);

  return [
    'payload_json' => $payload,
    'signature' => $signature
  ];
}

$ap_info = create_checkout_session_payload();

?>

<h1>Amazon Pay POC</h1>
<div id="AmazonPayButton"></div>

<script src="https://static-eu.payments-amazon.com/checkout.js"></script>
<script type="text/javascript" charset="utf-8">
  const amazonPayButton = amazon.Pay.renderButton('#AmazonPayButton', {
    // set checkout environment
    merchantId: 'merchant_id',
    publicKeyId: '<?= $GLOBALS['public_key'] ?>',
    ledgerCurrency: 'EUR',
    // customize the buyer experience
    checkoutLanguage: 'en_GB',
    productType: 'PayAndShip',
    placement: 'Cart',
    buttonColor: 'Gold',
    estimatedOrderAmount: { "amount": "109.99", "currencyCode": "EUR" },
    // configure Create Checkout Session request
    createCheckoutSessionConfig: {
      payloadJSON: '<?= $ap_info['payload_json'] ?>',
      signature: '<?= $ap_info['signature'] ?>',
      algorithm: 'AMZN-PAY-RSASSA-PSS-V2'
    }
  });
</script>