Give affiliates one single referral only.

affwp_custom_first_referral_only.php

<?php
/**
 * Plugin Name: AffiliateWP - First Referral Commission Only
 * Plugin URI: https://affiliatewp.com
 * Description: Modify an affiliate's commission rate for their very first referral. All other subsequent referrals will be recorded at zero.
 * Author: ramiabraham
 * Author URI: https://affiliatewp.com
 * Version: 1.0
 */



/**
 * Give the affiliate a $5 commission, but only for their very first referral.
 */
function affwp_first_ref_gets_five_else_zilch( $referral_amount, $affiliate_id, $amount, $reference, $product_id ) {
	
	// get the affiliate's referrals
	$referrals = affiliate_wp()->referrals->get_referrals( array( 'affiliate_id' => $affiliate_id ) );

	// if they haven't made a referral before, give them $5.00.
	if ( empty( $referrals ) ) {
		$referral_amount = 5.00;
	} else {
		$referral_amount = 0.00;
	}

	return $referral_amount;
}
add_filter( 'affwp_calc_referral_amount', 'affwp_first_ref_gets_five_else_zilch', 10, 5 );