how can I charge just 20% of the cart total at the gateway

As the title suggests, how can I charge just 20% of the cart total at the gateway?

If the cart total is $100, when the user click on buy now using a gateway such as paypal, the fee to pay in paypal is only $20 (ie: 20%) not $100.

I mean without applying discount vouchers etc... By actually modifying the code.

Anyone here have a clue?

Total-Based Fee/Discount can do this. All you do is set up a Charge with a value like this:

20%

Feel free to take a look at the screenshots and demo site, and if you're interested let me know at www.getclearthinking.com/contact if you have any further questions.