affwp_one_time_flat_rate_commission.php

<?php

function affwp_one_time_flat_rate_commission( $args, $amount, $order_id, $description, $affiliate_id, $visit_id, $products, $context ) {

	if ( ! function_exists( 'affiliate_wp' ) ) {
		return $args;
	}

	if ( 'flat' === affwp_get_affiliate_rate_type( $affiliate_id ) ) {
		$args['amount'] = (float) affwp_get_affiliate_rate( $affiliate_id );
	}

	return $args;

}
add_filter( 'affwp_insert_pending_referral', 'affwp_one_time_flat_rate_commission', 10, 8 );