Well someone said something about the donation button earlier.
But you can also make a "micro payment button" that acts pretty much like the donate button.
In fact it is in the "seller" part of paypal I think. I'm not sure, I haven't looked at this part for a long time, but you can setup an item (whatever the item is) as a button, and put the HTML code on your page/in your app.
It should send to a paypal page and once the payment is done, you maybe can set some redirection page.
You can do about the same with service like allopass.com
kiyoshi as answer for the question "how to monetize a game" you asked in another thread.
Micro payment is the key. Build your community around your game on your website and manage an item shop. That's your best bet and take full advantage of the online nature of HTML5.