This gist is used to create custom fields in the checkout form of EDD (Easy Digital Downloads).

add_custom_field_checkout_edd.php

<?php
/**
 * This gist is used to create custom fields in the
 * checkout form of EDD (Easy Digital Downloads).
 */

/**
 * Adding HTML for User Contact Form.
 */
function add_address_html() {
	?>
	<p id="edd-address-wrap">
		<label class="edd-label" for="edd-address">
			<?php _e( 'Delivery Address', 'edd' ); ?>
			<span class="edd-required-indicator">*</span>
		</label>
		<span class="edd-description">
			<?php _e( 'We will deliver this product to this address.', 'edd' ); ?>
		</span>
		<textarea class="edd-input required" rows="6" name="edd_address" id="edd-address" placeholder="<?php _e( 'Delivery Address', 'edd' ); ?>" value=""></textarea>
	</p>	
	<?php
}
add_action( 'edd_purchase_form_user_info', 'add_address_html' );

/**
 * Make address field required.
 */
function make_address_required( $required_fields ) {
	$required_fields 	= array(
		'edd_address' 		=> array(
			'error_id' 		=> 'invalid_address',
			'error_message'	=> 'Please enter a valid delivery address.'
		),
	);
	return $required_fields;
}
add_filter( 'edd_purchase_form_required_fields', 'make_address_required' );

/**
 * Check if address field is empty.
 */
function check_address_empty( $valid_data, $data ) {
	if ( empty( $data['edd_address'] ) ) {
        edd_set_error( 'invalid_address', 'Please enter your delivery address.' );
    }
}
add_action( 'edd_checkout_error_checks', 'check_address_empty', 10, 2 );

/**
 * Store the address in the payment meta.
 */
function store_address_meta( $payment_meta ) {
	$payment_meta['address'] = isset($_POST['edd_address']) ? $_POST['edd_address'] : '';
	return $payment_meta;
}
add_filter( 'edd_payment_meta', 'store_address_meta' );

/**
 * Show the address in the "View Order Details" popup.
 */
function address_edit_purchase_details( $payment_meta, $user_info ) {
	$address = isset( $payment_meta['address'] ) ? $payment_meta['address'] : 'none';
	?>
	<div class="column-container customer-info">
		<div class="column">
			<p><strong><?php echo __( 'Delivery Address:', 'pippin' ); ?></strong></p>
			<p><?php printf( '%s', $address ); ?></p>
		</div>
	</div>
	<?php
}
add_action( 'edd_payment_personal_details_list', 'address_edit_purchase_details', 10, 2 );