protected function getPaymentChannels($channel)
{
if (!isset($this->channels[$channel]) && $channel !== Gopay::METHOD_USER_SELECT) {
throw new InvalidArgumentException("Payment channel '{$channel}' is not supported");
}
if ($this->changeChannel === TRUE) {
$channels = array_keys($this->channels);
} else {
$channels = [$channel];
}
return $channels;
}