Adding additional items at checkout using User Fields

pmpro-additional-fees-checkbox.php

<?php 
/**
 * This recipe will add additional fees and pricing
 * 
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */
 
function my_pmpro_add_user_fields()	{
	// Don't break if PMPro is out of date or not loaded.
	if ( ! function_exists( 'pmpro_add_user_field' ) ) {
		return false;
	}

	// Store our field settings in an array.
	$fields = array();
	
	$fields[] = new PMPro_Field(
		'addon_options', // input field name, used as meta key
		'checkbox_grouped', // field type
		array(
			'label'          => 'Choose your Add Ons',
			'options'        => array( // display the different options, no need for a "blank" option
				'add_on_a' => 'I would like to receive Item 1: US $20',
				'add_on_b' => 'I would like to receive Item 2: US $40',
			),
			'profile'        => true,
			'memberslistcsv' => true,
			'addmember'      => true,
			'required'       => false,
		)
	);

	// Add a field group to put our fields into.
	pmpro_add_field_group( 'Add Ons' ); 

	//add the fields into a new checkout_boxes area of the checkout page
	foreach($fields as $field) {
		pmpro_add_user_field(
			'Add Ons',			// location on checkout page
			$field				// PMPro_Field object
		);
	}
}
add_action( 'init', 'my_pmpro_add_user_fields' );
 
function my_pmpro_checkout_level($level) {
	$addon_checkboxes = !empty( $_REQUEST['addon_options'] ) ? $_REQUEST['addon_options'] : '';
	if ( ! is_array( $addon_checkboxes ) ) {
		return $level;
	}
	foreach( $addon_checkboxes as $options ) {
		switch( $options ) {
			case "add_on_a":
				$level->initial_payment = $level->initial_payment + 20; //to update the initial payment.
				//$level->billing_amount = $level->billing_amount + 49;	//to update recurring payments too
			break;
			case "add_on_b":
				$level->initial_payment = $level->initial_payment + 40; //to update the initial payment.
				//$level->billing_amount = $level->billing_amount + 99;	//to update recurring payments too
			break;
		}
	}
	return $level;
}
add_filter( 'pmpro_checkout_level', 'my_pmpro_checkout_level' );