Add a charge to WooCommerce when the customer checks out via PayPal: https://pluginrepublic.com/woocommerce-payment-gateway-based-fees/

woocommerce-payment-gateway-fee.php

<?php
/**
 * Add a fee when the user checks out with PayPal
 */
function wcfad_apply_payment_gateway_fee() {
  $payment_method = WC()->session->get( 'chosen_payment_method' );
  // Only apply the fee if the payment gateway is PayPal
  // Note that you might need to check this slug, depending on the PayPal gateway you're using
  if( $payment_method == 'ppec_paypal' ) {
    $label = __( 'PayPal fee', 'wcfad' );
    $amount = 5; // Change this value to whatever amount you wish
    // Change the third parameter to false if you don't wish to apply tax to the fee
    // Change the fourth parameter to a different tax class if required
    WC()->cart->add_fee( $label, $amount, true, 'standard' );
  }
}
add_action( 'woocommerce_cart_calculate_fees', 'wcfad_apply_payment_gateway_fee' );

/**
 * Add some JS
 */
function wcfad_script() {
  ?>
  <script>
  jQuery(document).ready(function($){
    $('body').on('change','.checkout .input-radio',function(){
      $('body').trigger('update_checkout');
    });
  });
  </script>
<?php
}
add_action( 'woocommerce_after_checkout_form', 'wcfad_script' );

where add this file or code?

Please see https://pluginrepublic.com/woocommerce-payment-gateway-based-fees/