check-deposits.php

<?php
/**
 * Add the SSN field to the checkout if Four Easy Payments is available
 */
add_action( 'woocommerce_after_order_notes', 'my_custom_checkout_field' );

function my_custom_checkout_field( $checkout ) {
    if( has_deposit() ) {
        echo '<div id="my_custom_checkout_field"><h2>' . __('Payment Plan Requirements') . '</h2>';
        woocommerce_form_field('ssn_easypay', array(
            'type' => 'text',
            'label' => __('Payment Plans Require SSN'),
            'placeholder' => __('Social Security Number'),
        ), $checkout->get_value('ssn_easypay'));

        echo '</div>';
    }
}

/**
 * Process the checkout with SSN
 */
add_action('woocommerce_checkout_process', 'my_custom_checkout_field_process');

function my_custom_checkout_field_process() {
    if( has_deposit() ) {
        // Check if set, if its not set add an error.
        if ( ! $_POST['ssn_easypay'] || ! preg_match( '#^(\d{3})-(\d{2})-(\d{4})$#', $_POST['ssn_easypay'] ) )
            wc_add_notice( __( 'Please enter a valid SSN in the 111-22-3333 format and include the dashes.' ), 'error' );
    }
}

function has_deposit() {
    foreach ( WC()->cart->get_cart() as $cart_item ) {
        if ( ! empty( $cart_item['is_deposit'] ) ) {
            return true;
        }
    }
    return false;
}
?>