Always show payment methods

If you have only one payment method, then Drupal Commerce 2.x will hide it. We can change that behavior in the form alter hook.

use Drupal\Core\Form\FormStateInterface;

function MY_MODULE_form_alter(&$form, FormStateInterface $form_state, $form_id) {
  if ($form_id == 'commerce_checkout_flow_multistep_default') {
    if (isset($form['payment_information']['payment_method']['#access'])) {
      $form['payment_information']['payment_method']['#access'] = TRUE;
    }
  }
}