adding tax to paypal payment method PS 1.4.3

[Arepie]

Hello,

As you all know, paypal took some % from every payment. So i want to cover up the tax to the customer. So for me, i add (4.5% + 2). This amount will be close enough to the total amount.

So what i did, i open up paypal/payment/paypalpayment.php and edit

line 52:        $paymentAmount = (float)(($cart->getOrderTotal() * 1.045) + 2);

Now at the confirmation page, it display the correct amount (total + shipping) + (4.5% + 2). But after i confirm the order, i receive

Authorisation to PayPal failed
Please refer to logs:

   Making new connection to 'api-3t.paypal.com/nvp'
   Connect with CURL method successful
   Sending this params:
   SetExpressCheckout
   Send with CURL method successful

After doing some research, i found that the payment to paypal is not equal to the total in prestashop cart, which stop me from continue.

I tried to find many solutions, but i found nothing. I stuck at this for at least 4 days figuring how to add the tax amount to the cart.

Please help me find the solutions.

Thank you.