edd_convertkit_complete_purchase.php

<?php

/***
	Update Convertkit email with a Convertkit tag based on EDD download ID
	
	Conditionally set Convertkit tag ID based on EDD download ID
	
***/

add_action( 'edd_complete_purchase', 'drw_update_ck_after_edd_purchase' );

function drw_update_ck_after_edd_purchase( $payment_id ) {
	
	$edd_payment = new EDD_Payment($payment_id);
	
	$email = $edd_payment->email;
	
	$cart_items = edd_get_payment_meta_cart_details( $payment_id );

	foreach ( $cart_items as $download ) {
		
		$download_id = $download['id'];
		
		if ( 321 == $download_id ) {
			
			$ck_tag_id = 4879234;
			
			drw_update_ck_subscriber( $email, $ck_tag_id, 'subscribe' );
			
		} elseif( 430 == $download_id ) {
			
			$ck_tag_id = 4879234;
			
			drw_update_ck_subscriber( $email, $ck_tag_id, 'subscribe' );
		
		}
	}
}

edd_convertkit_update_ck.php

<?php

/***
	Define Convertkit API key
***/

define('CK_API_KEY', 'your_convertkit_api_key_here');

/***
	Send subscriber data to Convertkit
	
	$email		str		customer email address to send to CK

	$endpoint	str		'subscribe', 'unsubscribe'

***/

function drw_update_ck_subscriber( $email, $ck_tag_id, $endpoint = null ) {

	if ( null == $endpoint ) $endpoint = 'subscribe';
	
	//Add extra fields here if necessary
	$args = array(
		'email' => $email,
	);

	$request = wp_remote_post(
		'https://api.convertkit.com/v3/tags/' . $ck_tag_id . '/'.$endpoint.'?api_key='.CK_API_KEY,
		array(
			'body'    => $args,
			'timeout' => 15,
		)
	);

	if( ! is_wp_error( $request ) && 200 == wp_remote_retrieve_response_code( $request ) ) {

		$return = true;

	}
}