This order-total module allows the customer to specify "priority handling," and allows you to add an appropriate service charge.
Requires Zen Cart 2.1.0 (or later), PHP 8.0 or later
Zen Cart support-thread link: https://www.zen-cart.com/showthread.php?50420-Rush-Priority-Handling-Charge
Zen Cart download link: https://www.zen-cart.com/downloads.php?do=file&id=236