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Not really. Bittrex and Polo are able to process my deposit if I encrypted memo in the standard way. Currently memo is not encrypted by default on Steemit.com, but already some people asking for "encrypt memo by default", if it's applied, you need to warn the users to NOT encrypt memo, because users may not even know it's encrypted by default. Yes it's just some concerns about future.

What if I want to donate some to arhag? still send my fund to steve-walschot?

You change the receiver to arhag. For example in raw URL

https://steempay.io/payment?&receiver=arhag&amount=0&currency=0&callback=http://www.steemit.com

I mean, lack of a "to" field in the page. If I send fund to arhag directly, how can you process?

Because every action is recorded in the blockchain. It will process your donation to arhag. You can also set your receiver in the generator, click generate and then click the button to go straight to the page

OK, I understand now. But you will be unable to decrypt encrypted memo.

Using following memo code If you change the memo code, Steempay will not be able to process it. Just like you would deposit your coins at bittrex/polo and mistyping your memo code. The receiver will receive the coins, but steempay will not pickup the transaction (yet)

https://steempay.io/payment/www.steemit.com?payid=mTv41AjYsDQmubqQ2A
Got an exception when clicking on the Go back to your merchant link, some code is printed out.

Symfony \ Component \ HttpKernel \ Exception \ NotFoundHttpException

Open: /var/dev_steempay/bootstrap/compiled.php

        if (!is_null($route)) {
            return $route->bind($request);
        }
        $others = $this->checkForAlternateVerbs($request);
        if (count($others) > 0) {
            return $this->getOtherMethodsRoute($request, $others);
        }
        throw new NotFoundHttpException();
    }
    protected function checkForAlternateVerbs($request)

Perhaps caused by lack of http:// on callback in your first version of the reply.

Set the callback parameter with your desired url. Always use http:// to start your url.