getpaid-percent-fee.php

<?php

add_action(
    'getpaid_submissions_process_items',
    function( &$submission ) {
        /**@var GetPaid_Payment_Form_Submission $submission */
        $percent = 5 / 100; // 5% processing fee.

        $submission->remove_fee( 'Processing Fee' );
        $submission->add_fee(
            array(
                'name'          => 'Processing Fee',
                'initial_fee'   => $submission->get_subtotal() * $percent,
                'recurring_fee' => $submission->get_recurring_subtotal() * $percent,
            )
        );

        return $submission;
    }
);