Always show payment methods

Components

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;
    }
  }
}