Adds a percentage surcharge line item to an Event Espresso transaction, the percentage will be based on the answer of a question on the registration form. Variation of https://github.com/eventespresso/ee-code-snippet-library/blob/master/checkout/bc_ee_apply_transaction_surcharge.php

percentage_surcharge.php

<?php
/**
 * PLEASE READ AND FOLLOW ALL INSTRUCTIONS IN CAPS
 *
 * IN ORDER FOR THIS TO WORK YOU NEED TO ADD A CUSTOM QUESTION
 * BY LOGGING INTO THE WORDPRESS ADMIN AND GOING TO :
 *        Event Espresso > Registration Form
 * AND THEN CLICKING ON "Add New Question"
 * FOR THIS EXAMPLE CODE I CREATED A QUESTION NAMED "Billing Address Province"
 * SET ITS TYPE TO "Dropdown" AND GAVE IT THE FOLLOWING TWO OPTIONS:
 * 		"Ontario"
 * 		"Alberta"
 * (THE ANSWER VALUES ARE CASE SENSITIVE)
 * THEN SET THE QUESTION TO REQUIRED
 *
 * BECAUSE THIS QUESTION SHOULD ONLY BE ASKED ONCE PER TRANSACTION
 * I ALSO CREATED A QUESTION GROUP CALLED "Billing Contact Info"
 * AND ADDED THE "Billing Address Province" QUESTION TO THAT GROUP
 *
 * THEN ON MY EVENT ( Event Espresso > Events > Edit Event ),
 * I CHECKED OFF THE  "Billing Address Province" QUESTION GROUP
 * IN THE "Questions for Primary Registrant" SIDEBAR METABOX
 *
 * THIS WAY, ONLY THE PRIMARY REGISTRANT WILL BE ASKED
 * TO SELECT A BILLING ADDRESS PROVINCE, WHICH WILL THEN
 * CONTROL WHICH CHARGE IS ADDED TO THE TRANSACTION
 *
 * PLZ SEE ADDITIONAL INSTRUCTIONS IN FUNCTIONS BELOW
 *
 * bc_ee_determine_whether_to_apply_surcharge
 *
 * @return void
 */
 
 
function bc_ee_determine_whether_to_apply_surcharge() {
  // CHANGE $surcharge_QST_ID VALUE TO MATCH THE ID OF YOUR QUESTION
  $surcharge_QST_ID = 119;
  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 ) {
              // CHANGE THESE TO EXACTLY MATCH THE ANSWER OPTIONS FOR YOUR QUESTION
              // THEN EDIT / ADD / DELETE THE FUNCTIONS BELOW
              // WHOSE NAMES MATCH THESE OPTIONS
              // YOU CAN ADD NEW OPTIONS, JUST MAKE SURE TO HAVE A CORRESPONDING
              // FUNCTION FOR SETTING THE SURCHARGE DETAILS
              case 'Ontario' :
                // 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__bc_ee_apply_transaction_surcharge__surcharge_details', 'ee_ontario_surcharge_details' );
                break;
              case 'Alberta' :
                // 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__bc_ee_apply_transaction_surcharge__surcharge_details', 'ee_alberta_surcharge_details' );
                break;
            }
          }
        }
      }
    }
  }
}
add_action( 'AHEE__EE_System__core_loaded_and_ready', 'bc_ee_determine_whether_to_apply_surcharge', 1 );
/**
 * EDIT THIS TO HOLD THE DETAILS FOR ONE OF YOUR ANSWER OPTIONS
 * @return array
 */
function ee_ontario_surcharge_details() {
  return array(
    'name'          => 'ontario hst',
    'code'          => 'ontario-hst',
    'description'   => 'ontario hst 13%',
    'percent'       => 13,
    'taxable'       => false,
  );
}
/**
 * EDIT THIS TO HOLD THE DETAILS FOR ONE OF YOUR ANSWER OPTIONS
 * @return array
 */
function ee_alberta_surcharge_details() {
  return array(
    'name'          => 'alberta hst',
    'code'          => 'alberta-hst',
    'description'   => 'alberta hst 5%',
    'percent'       => 5,
    'taxable'       => false,
  );
}
/**
 * DO NOT EDIT ANYTHING EXCEPT DEFAULT SURCHARGE DETAILS
 *
 * bc_ee_apply_transaction_surcharge
 *
 * @param \EE_Checkout $checkout
 * @return \EE_Checkout
 */
function bc_ee_apply_transaction_surcharge( EE_Checkout $checkout ) {
  // DEFAULT SURCHARGE DETAILS - EDIT THIS
  $surcharge_details = apply_filters(
    'FHEE__bc_ee_apply_transaction_surcharge__surcharge_details',
    array(
      //  name for surcharge that will be displayed
      'name'        => 'gst',
      // unique code used to identify surcharge in the db
      'code'        => 'province-gst',
      // description for line item
      'description' => 'province gst',
      // percentage amount
      'percent'     => 5,
      // whether or not tax is applied on top of the surcharge
      'taxable'     => false,
    )
  );
  // STOP EDITING
  // 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' ],
    ) )
  );
  $grand_total->recalculate_total_including_taxes();
  return $checkout;
}
add_filter( 'FHEE__EED_Single_Page_Checkout___initialize_checkout__checkout', 'bc_ee_apply_transaction_surcharge' );

Hi, I just noticed an issue when a student would register with a pending payment, and change the province after the fact, it would add the new tax code to the invoice but not remove the old province's tax code. Any suggestion to fix that?