/**
  * Gets saved tokens from API if they don't already exist in WooCommerce.
  * @param array $tokens
  * @return array
  */
 public function woocommerce_get_customer_payment_tokens($tokens, $customer_id, $gateway_id)
 {
     if (is_user_logged_in() && 'stripe' === $gateway_id && class_exists('WC_Payment_Token_CC')) {
         $stripe_customer = new WC_Stripe_Customer($customer_id);
         $stripe_cards = $stripe_customer->get_cards();
         $stored_tokens = array();
         foreach ($tokens as $token) {
             $stored_tokens[] = $token->get_token();
         }
         foreach ($stripe_cards as $card) {
             if (!in_array($card->id, $stored_tokens)) {
                 $token = new WC_Payment_Token_CC();
                 $token->set_token($card->id);
                 $token->set_gateway_id('stripe');
                 $token->set_card_type(strtolower($card->brand));
                 $token->set_last4($card->last4);
                 $token->set_expiry_month($card->exp_month);
                 $token->set_expiry_year($card->exp_year);
                 $token->set_user_id($customer_id);
                 $token->save();
                 $tokens[$token->get_id()] = $token;
             }
         }
     }
     return $tokens;
 }