How to add Custom Data to Cart in EDD | http://www.ibenic.com/add-custom-data-cart-easy-digital-downloads

add_item.php

<?php

add_filter( 'edd_add_to_cart_item', 'dp_check_item_to_add' );

/**
 * Check the item we want to add in cart (Step #7)
 * @param  array $item 
 * @return array|boolean       
 */
function dp_check_item_to_add( $item ) {

	if( isset( $item['id'] ) ) {
		
		$download_id = dp_get_download_id();

		// Our Download <-> Questionnaire Product is the item
		if( $download_id && $download_id == $item['id'] ) {

			if( isset( $_REQUEST['questionnaire_id'] ) ) {
				$item['options']['questionnaire_id'] = $_REQUEST['questionnaire_id'];
			}

			// We already have this one
			if( edd_item_in_cart( $item['id'], $item['options'] ) ) {
				return false;
			}
			
		}
	}

	return $item;
}

check_item.php

<?php

add_filter( 'edd_item_in_cart', 'dp_item_in_cart', 20, 3 );

/**
 * Return if the Item is already in the cart
 * @param  boolean $return      
 * @param  integer $download_id 
 * @param  array $options     
 * @return boolean              
 */
function dp_item_in_cart( $return, $download_id, $options ) {

	// The item is already in the cart
	if( $return ) {
		$dp_download_id = dp_get_download_id();

		// This is our Worksheet item
		if( $dp_download_id && $dp_download_id == $download_id ) {
			$cart = edd_get_cart_contents();

			if ( is_array( $cart ) ) {
				foreach ( $cart as $item ) {
					if ( $item['id'] == $download_id ) {
						if ( isset( $options['questionnaire_id'] ) && isset( $item['options']['questionnaire_id'] ) ) {
							if ( $options['questionnaire_id'] == $item['options']['questionnaire_id'] ) {
								// Yep, same Questionnaire ID for the product
								$return = true;
								break;
							} else {
								// The Questionnaire IDs are not the same
								$return = false;
								break;
							}
						} else {
							$return = false;
							break;
						}
					}
				}
			}
		}
	}

	return $return;
}

complete.php

<?php

add_action( 'edd_complete_download_purchase', 'dp_item_download_complete', 20, 5 );

/**
 * Set the Questionnaire as paid when the purchase is completed
 */
function dp_item_download_complete( $download_id, $payment_id, $download_type, $download, $cart_index ) {
 
	if( isset( $download['item_number']['options']['questionnaire_id'] ) ) {

		$questionnaire = get_post( $download['item_number']['options']['questionnaire_id'] );

		// Check if it is not null
		if( $questionnaire ) {

			 // Do something

		}
	}
}