edit after end page using paypal

Hi,

1. I'm sorry but I don't understand what you mean by "Want to alert message".

2. The message there already comes from the after_end.php view file. Now, if you want to display the message in the main area of your template instead of the message system of Joomla, you need to change that after_end.php and instead of sending the text to the messaging system, you can just echo the text directly there.

3. I suppose that you're talking about the URL in the address bar when you reach this page. Actually, that URL is usually provided by the payment plugin used for the payment. So what to do will depend on the payment plugin.
Now an easy solution would be to create a page with the content you want to display in a joomla article and enter the URL of that article in the "return URL" of your payment method. That way, you would see that page after the payment instead of the default one which would I suppose fix all your questions here.