Only charge 20% of cart total at the checkout

I can’t supply the URL as the site does not belong to me, but if you can answer without it, I’d appreciate any pointers in the right direction.

RE: get_cart_total();

As the title suggests, I’m trying (in vain) to only charge customers just 20% of the cart total at the checkout, using any gateway method.

eg: Let’s say the cart total adds up to $100.

If a customer decides to buy, they only have to pay 20% of the cart total upfront.

So if they click on the buy now button and get directed to whichever gateway is being used (eg: paypal) the fee in the paypal will $20 ie: 20% of $100.

Hope you understand what I am trying to achieve. If do not hesitate to ask for more info.

Thanks