Call to undefined method Drupal\commerce_payment\PaymentOptionsBuilder::t()

We need to inject $string_translation, and add the missing test coverage.

Done.

Why do you inject the string translation manager, but use the trait then? (btw, it's recommended to use the trait anyway regarding docs)

Because the trait provides an API not provided by the service:

  protected function t($string, array $args = [], array $options = []) {
    return new TranslatableMarkup($string, $args, $options, $this->getStringTranslation());
  }

  protected function formatPlural($count, $singular, $plural, array $args = [], array $options = []) {
    return new PluralTranslatableMarkup($count, $singular, $plural, $args, $options, $this->getStringTranslation());
  }

It's odd, I know.

true...wow.. didn't know that