Adds surcharge line items for those that choose online payments.

joe.php

<?php
/*
Plugin Name: Event Espresso add custom online payments surcharges
Plugin URI: https://eventespresso.com/topic/is-there-a-way-to-add-surcharge-only-if-question-checkbox-is-checked/
Description: If answer A then fee remains the same. If answer B then additional charges apply.
Version: 1.0
Author: Event Espresso
Author URI: http://www.eventespresso.com
*/

function bc_ee_determine_whether_to_apply_surcharge() {
  $surcharge_QST_ID = 91;
  if ( isset( $_REQUEST[ 'ee_reg_qstn' ] ) ) {
    foreach ( $_REQUEST[ 'ee_reg_qstn' ] as $registrations ) {
      if ( ! empty( $registrations ) ) {
        foreach ( $registrations as $QST_ID => $response ) {
          if ( $QST_ID === $surcharge_QST_ID ) {
            switch ( $response ) {
              case 'E-Check' :
                // don't apply the surcharge
                break;
              case 'Credit/Debit Card' :
                // apply the surcharge
                add_filter( 'FHEE__bc_ee_apply_transaction_surcharge__apply_surcharge', '__return_true' );
                // hook into function below to set surcharge details
                add_filter( 'FHEE__EED_Single_Page_Checkout___initialize_checkout__checkout', 'bc_ee_apply_transaction_surcharge', 10 );
                break;
            }
          }
        }
      }
    }
  }
}
add_action( 'AHEE__EE_System__core_loaded_and_ready', 'bc_ee_determine_whether_to_apply_surcharge', 1 );

function bc_ee_apply_transaction_surcharge( EE_Checkout $checkout ) {
  $surcharge_details = array(
    'name'          => 'Transaction Fee',
    'code'          => 'percent-surcharge-fee',
    'description'   => 'convenience fee for online payments',
    'percent'       => 3.4,
    'unit_price'    => .25,
    'taxable'       => false,
  );
  // apply the surcharge ?
  if ( ! apply_filters( 'FHEE__bc_ee_apply_transaction_surcharge__apply_surcharge', false ) ) {
    return $checkout;
  }
  // verify checkout
  if ( ! $checkout instanceof EE_Checkout ) {
    return $checkout;
  }
  // verify cart
  $cart = $checkout->cart;
  if ( ! $cart instanceof EE_Cart ) {
    return $checkout;
  }
  // verify grand total line item
  $grand_total = $cart->get_grand_total();
  if ( ! $grand_total instanceof EE_Line_Item ) {
    return $checkout;
  }
  // has surcharge already been applied ?
  $existing_surcharge = $grand_total->get_child_line_item( $surcharge_details[ 'code' ] );
  if ( $existing_surcharge instanceof EE_Line_Item ) {
    return $checkout;
  }
  EE_Registry::instance()->load_helper( 'Line_Item' );
  $pre_tax_subtotal = EEH_Line_Item::get_pre_tax_subtotal( $grand_total );
  $pre_tax_subtotal->add_child_line_item(
    EE_Line_Item::new_instance( array(
      'LIN_name'       => $surcharge_details[ 'name' ],
      'LIN_desc'       => $surcharge_details[ 'description' ],
      'LIN_unit_price' => 0,
      'LIN_percent'    => $surcharge_details[ 'percent' ],
      'LIN_quantity'   => NULL,
      'LIN_is_taxable' => $surcharge_details[ 'taxable' ],
      'LIN_order'      => 0,
      'LIN_total'      => (float) ( $percentage_amount * ( $pre_tax_subtotal->total() / 100 ) ),
      'LIN_type'       => EEM_Line_Item::type_line_item,
      'LIN_code'       => $surcharge_details[ 'code' ],
    ) )
  );
  $pre_tax_subtotal->add_child_line_item(
    EE_Line_Item::new_instance( array(
      'LIN_name'       => $surcharge_details[ 'name' ],
      'LIN_desc'       => $surcharge_details[ 'description' ],
      'LIN_unit_price' => (float) $surcharge_details[ 'unit_price' ],
      'LIN_quantity'   => 1,
      'LIN_is_taxable' => $surcharge_details[ 'taxable' ],
      'LIN_order'      => 0,
      'LIN_total'      => (float) $surcharge_details[ 'unit_price' ],
      'LIN_type'       => EEM_Line_Item::type_line_item,
      'LIN_code'       => 'flat-fee',
    ) )
  );
  $grand_total->recalculate_total_including_taxes();
  return $checkout;
}