"forked" from https://github.com/eventespresso/ee-code-snippet-library/blob/master/checkout/bc_add_cart_modifier.php

bc_add_cart_modifier.php

<?php
/**
 * bc_add_cart_modifier
 *
 * @param \EE_SPCO_Reg_Step $payment_options_reg_step
 * @throws \EE_Error
 */
function bc_add_cart_modifier( EE_SPCO_Reg_Step $payment_options_reg_step ) {
	// CHANGE THESE TO YOUR LIKING
	$cart_modifier_name = 'my cart % surcharge';
	$cart_modifier_amount = 10.00;
	$cart_modifier_description = '10% surcharge for choosing invoice payment method';
	$cart_modifier_taxable = false; // or false if surcharge is not taxable
	$payment_methods_with_surcharges = array( 'square' );
	
	// get what the user selected for payment method
	$selected_method_of_payment = $payment_options_reg_step->checkout->selected_method_of_payment;
	$cart = $payment_options_reg_step->checkout->cart;
	if ( ! $cart instanceof EE_Cart ) {
		// ERROR
		return;
	}
	$total_line_item = $cart->get_grand_total();
	if ( ! $total_line_item instanceof EE_Line_Item && ! $total_line_item->is_total() ) {
		// ERROR
		return;
	}
	$transaction = $payment_options_reg_step->checkout->transaction;
	if ( ! $transaction instanceof EE_Transaction) {
		// ERROR
		return;
	}
	//delete existing in case page is refreshed or something
	EEM_Line_Item::instance()->delete(
		array(
			array(
				'TXN_ID'   => $transaction->ID(),
				'LIN_name' => $cart_modifier_name,
			)
		)
	);
	$amount_owing_before = $payment_options_reg_step->checkout->amount_owing;

	// check payment type here instead to remove surcharge when not using square
	if ( in_array( $selected_method_of_payment, $payment_methods_with_surcharges, true ) ) {
		$surcharge_line_item_id = EEH_Line_Item::add_percentage_based_item(
			$total_line_item,
			$cart_modifier_name,
			$cart_modifier_amount,
			$cart_modifier_description,
			$cart_modifier_taxable
		);
	}
//	if ($surcharge_line_item_id) {
		$total_line_item->recalculate_total_including_taxes();
		$payment_options_reg_step->checkout->amount_owing += ($total_line_item->total() - $amount_owing_before);
		$transaction->set_total($total_line_item->total());
		$success = $transaction->save();
		if ($success) {
			/** @type EE_Registration_Processor $registration_processor */
			$registration_processor = EE_Registry::instance()->load_class('Registration_Processor');
			$registration_processor->update_registration_final_prices($transaction);
		}
//	}
}

// the switch_payment_method hook makes more sense
add_action( 'AHEE__Single_Page_Checkout__before_payment_options__switch_payment_method', 'bc_add_cart_modifier', 10, 1 );
//add_action( 'AHEE__Single_Page_Checkout__before_payment_options__process_reg_step', 'bc_add_cart_modifier', 10, 1 );

// Reload the page once a payment method has been switched.
function tw_ee_refresh_checkout_on_payment_method_change() {
  wp_add_inline_script( 
    'single_page_checkout',
    'jQuery(function () {
        SPCO.main_container.on( \'spco_switch_payment_methods\', function() { 
            window.location.reload(); 
        });
    });'
  );
}
add_action( 'wp_enqueue_scripts', 'tw_ee_refresh_checkout_on_payment_method_change', 11 );