Well it involves the same as some of the other scams, as in changing the actual value of your order.
There has been talk about changing the value from like 50$ to 5.0$ or 0.50$
But you can even alter it a bit more...
In the HTML code before the paypal payment (right click in the page, press view source), in the paypal form it says
<INPUT type=hidden value=USD name=currency_code>
Now if you change USD to ITL (italian lira), and leave the value to 50.00,
you will still only pay like 0.12$ or something, and it will show as a 50.00 payment
If you've altered this, save the file as a .html file to your hard disk and execute it and finalize the payment.
Now i cant guarantee this will work on most sites, but if someone is willing to give it a try, please do & let us know the results
I cant do this myself atm because my paypal is limited and i need to fax some documents
If it doesnt work, i apologise & pelase delete this thread.