First of all, it's a rather large download, so you're probably going to close the website as soon as you click on the download button. Second of all, to consume the product in this case, you have to install it. And yet again, after you do so, if you get greeted with the "donate" window right away, you're going to dismiss it because you still haven't tested out the product.

So, what you need is a timer, something that says like "after 30 minutes (or an hour, or two hours) of usage, display the window", but that would require the OS to make calls to external servers without the user consent, and, because we are talking about a Linux-based operating system, people would be complaining that this is a a privacy violation.