There is a definite bug in your code (1 so far)

WooPickupShipping.php line 535 to 541
The variabel $selectedShift is used even if not set due to the conditions not being met, so should be defined beforehand.

 if (isset($_SESSION['picup_scheduled_selected_shift']) && !empty($_SESSION['picup_scheduled_selected_shift'])) {
            $selectedShift = sanitize_text_field($_SESSION['picup_scheduled_selected_shift']);

            $args['default'] = $selectedShift;
        }
$this->woocommerceAdapter->woocommerceFormField('picup_scheduled_shift', $args, $selectedShift);

This results in

Notice: Undefined variable: selectedShift in /var/www/wip.southcoasthosting.com/wp-content/plugins/woo-picup-shipping/src/WooPicupShipping.php on line 541