Add a greeting card field to the checkout page after the order notes.

greeting-card-custom-field.php

<?php
// Do NOT include the opening php tag.
// Place in your theme's functions.php file

/**
 * You can use the following example to add virtually any custom fields
 * to the checkout page. To more specifically control the location, use
 * our hook guide: https://kb.checkoutwc.com/article/25-actions
 */

/**
 * Add the field to the checkout page
 */
add_action( 'cfw_checkout_before_payment_method_terms_checkbox', 'customise_checkout_field' );

function customise_checkout_field() {
	woocommerce_form_field(
		'greeting_card',
		array(
			'type'        => 'textarea',
			'label'       => 'Greeting Card',
			'placeholder' => 'Greeting Card',
			'required'    => false,
			'class'       => array(),
			'autofocus'   => false,
			'priority'    => 10,
			'columns'     => 12,
		),
		WC()->checkout->get_value( 'customised_field_name' )
	);
}

/**
 * Update value of field
 */
add_action( 'woocommerce_checkout_update_order_meta', 'customise_checkout_field_update_order_meta' );
function customise_checkout_field_update_order_meta( $order_id ) {
	if ( ! empty( $_POST['greeting_card'] ) ) {
		update_post_meta( $order_id, 'greeting_card', sanitize_text_field( $_POST['greeting_card'] ) );
	}
}

/**
 * Display field value on the order edit page
 */
add_action( 'woocommerce_admin_order_data_after_billing_address', 'greeting_card_checkout_field_display_admin_order_meta', 10, 1 );
function greeting_card_checkout_field_display_admin_order_meta($order){
	echo '<p><strong>'.__('Greeting Card').':</strong> <br/>' . get_post_meta( $order->get_id(), 'greeting_card', true ) . '</p>';
}