An example of how to create a custom email tag in EDD 1.9+

gistfile1.php

<?php

class EDD_Sample_Email_Tag {

	function __construct() {

		add_action( 'edd_add_email_tags', array( $this, 'add_sample_tag' ), 100 );

	}

	public function add_sample_tag() {

		edd_add_email_tag( 'replace_with_your_tag', 'Put your tag description here', array( $this, 'render_tag_content' ) );

	}

	public function render_tag_content( $payment_id = 0 ) {

		$output  = '';
		$cart_items = edd_get_payment_meta_cart_details( $payment_id, true );
		
		foreach( $cart_items as $item ) {
			// do something to $output for each cart item
		}

		return $output;

	}


}
new EDD_Sample_Email_Tag;