Add a %fee to Total Cart -> Administration

[hellykun]

Dear Everyone,

 

I am using  the Universal Payments module and custom code in order to create a payment that adds 3% fee to the client.

I have one problem.

 

How am I supposed to add the fee to the $Total Cart variable?

 

So as to transfer the final cost (with the fee) to the prestashop administration.

 

Any help will be appreciated.