Change cart total price

[Nicolas Pistillo]

I everyone! im trying to change the final price to pay in the cart, I have a custom tax to show at te cart price lines and it works, but i need to change the final price to pay with this custom tax percentage added. 

Example:

3 articles                 --- $1500

Custom tax(1.3%)   ---  $19.5

TOTAL                     --- $1519.5

 

Is there any hook or way to add this custom tax at the cart object or change the payment object for achieve this? 

(yes, i don't have so much experience with presta, please be patient)