you should create 2 divs:

<div id="home">...</div> (here you have your sales message or confirmation code)
<div id="backup">..</div> (here you have your down-sale message code)

the second div has to be hidden --> style="display:none"

then you need to setup a JS script where you define the function:

window.onbeforeunload

this is a native JS function for the page. If you add code to this function something will happen when the user try to press the close button---> this trigger the onbeforeunload event of the current window.

Then in this code you need to open a message using the alert function alert("Please wait...");

then in this function you should now hide the first div and make visible the second...
something like:

window.onbeforeunload=askwhatsnext;

where askwhatsnext---->

function askwhatsnext(){
alert("please wait....");
document.getElementById("home").style.display="non e";
document.getElementById("backup").style.display="" ;
}

add this script at the bottom of your HTML code just before the tag </body>

Please let me know if you need more details