javascript - Opencart 1.5; How to add 15% to dollar amount passed to Authorize.net gateway -

i using opencart version 1.5 , authorize.net gateway. shipping , tax needs calculated after purchase need sure , authorize amount above purchase price. know how add percentage amount 'authorize' amount sent gateway? simpler past experience, guessing need intercept amount variable directly after customer clicks purchase @ checkout prior posting gateway.

to achieve need go below file:

catalog\controller\payment\authorizenet_*

search below code

$data['x_amount']

and add below line after above line:

$data['x_amount'] = $data['x_amount'] + ((15* $data['x_amount'])/100);