Add a checkbox on single product pages that adds a

In woocommerce, I am using the following code to display an input text field on admin product edit pages, that allow me to add an additonal pricing option to the product:

add_action( 'woocommerce_product_options_pricing', 'wc_cost_product_field' );
function wc_cost_product_field() {
    woocommerce_wp_text_input( array( 'id' => 'repair_price', 'class' => 'wc_input_price short', 'label' => __( 'Repair Cost', 'woocommerce' ) . ' (' . get_woocommerce_currency_symbol() . ')' ) );
}

add_action( 'save_post', 'wc_cost_save_product' );
function wc_cost_save_product( $product_id ) {

     // stop the quick edit interferring as this will stop it saving properly, when a user uses quick edit feature
     if (wp_verify_nonce($_POST['_inline_edit'], 'inlineeditnonce'))
        return;

    // If this is a auto save do nothing, we only save when update button is clicked
    if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE )
        return;
    if ( isset( $_POST['repair_price'] ) ) {
        if ( is_numeric( $_POST['repair_price'] ) )
            update_post_meta( $product_id, 'repair_price', $_POST['repair_price'] );
    } else delete_post_meta( $product_id, 'repair_price' );
}

Now on the product page (in the front end) I need a checkbox "Repair cost $20", that will appear when the product custom field pricing option is filled with some price.

If customer add this option, it will add an additional cost of $20 on the product and it should also be reflected on the cart and checkout pages.

Any help will be appreciated.

In the following code, I have revisited a bit your actual code and added all necessary code to enable the checkbox option in single product page if the pricing option in the product is filled (for non variable products type that are not on sale).

When the option is selected by the customer, the product price is increased and when added to cart the price change is reflected in cart and checkout pages (see the screenshots at the end).

The code:

// Backend: Additional pricing option custom field
add_action( 'woocommerce_product_options_pricing', 'wc_cost_product_field' );
function wc_cost_product_field() {
    woocommerce_wp_text_input( array(
        'id'        => '_repair_price',
        'class'     => 'wc_input_price short',
        'label'     => __( 'Repair Cost', 'woocommerce' ) . ' (' . get_woocommerce_currency_symbol() . ')'
    ));
}

// Backend: Saving product pricing option custom field value
add_action( 'woocommerce_admin_process_product_object', 'save_product_custom_meta_data', 100, 1 );
function save_product_custom_meta_data( $product ){
    if ( isset( $_POST['_repair_price'] ) )
        $product->update_meta_data( '_repair_price', sanitize_text_field($_POST['_repair_price']) );
}

// Front: Add a text input field inside the add to cart form on single product page
add_action('woocommerce_single_product_summary','add_repair_price_option_to_single_product', 2 );
function add_repair_price_option_to_single_product(){
    global $product;

    if( $product->is_type('variable') || ! $product->get_meta( '_repair_price' ) ) return;

    add_action('woocommerce_before_add_to_cart_button', 'product_option_custom_field', 30 );
}

function product_option_custom_field(){
    global $product;

    $active_price = (float) $product->get_price();
    $repair_price = (float) $product->get_meta( '_repair_price' );
    $repair_price_html   = strip_tags( wc_price( wc_get_price_to_display( $product, array('price' => $repair_price ) ) ) );
    $active_price_html   = wc_price( wc_get_price_to_display( $product ) );
    $disp_price_sum_html = wc_price( wc_get_price_to_display( $product, array('price' => $active_price + $repair_price ) ) );

    echo '<div class="hidden-field">
    <p class="form-row form-row-wide" id="repair_option_field" data-priority="">
    <span class="woocommerce-input-wrapper"><label class="checkbox"> ' . __("Repair Option:", "Woocommerce") .
    ' <input type="checkbox" class="input-checkbox " name="repair_option" id="repair_option" value="1"> + ' . $repair_price_html .
    '</label></span></p>
    <input type="hidden" name="repair_price" value="' . $repair_price . '">
    <input type="hidden" name="active_price" value="' . $active_price . '"></div>';

    // Jquery: Update displayed price
    ?>
    <script type="text/javascript">
    jQuery(function($) {
        var cb = 'input[name="repair_option"]'
            pp = 'p.price';

        // On change / select a variation
        $('form.cart').on( 'change', cb, function(){
            if( $(cb).prop('checked') === true )
                $(pp).html('<?php echo $disp_price_sum_html; ?>');
            else
                $(pp).html('<?php echo $active_price_html; ?>');
        })

    });
    </script>
    <?php
}

// Front: Calculate new item price and add it as custom cart item data
add_filter('woocommerce_add_cart_item_data', 'add_custom_product_data', 10, 3);
function add_custom_product_data( $cart_item_data, $product_id, $variation_id ) {
    if (isset($_POST['repair_option']) && !empty($_POST['repair_option'])) {
        $cart_item_data['new_price'] = (float) ($_POST['active_price'] + $_POST['repair_price']);
        $cart_item_data['repair_price'] = (float) $_POST['repair_price'];
        $cart_item_data['active_price'] = (float) $_POST['active_price'];
        $cart_item_data['unique_key'] = md5(microtime().rand());
    }

    return $cart_item_data;
}

// Front: Set the new calculated cart item price
add_action('woocommerce_before_calculate_totals', 'extra_price_add_custom_price', 20, 1);

function extra_price_add_custom_price($cart) {
    if (is_admin() && !defined('DOING_AJAX'))
        return;

    if ( did_action( 'woocommerce_before_calculate_totals' ) >= 2 )
        return;

    foreach($cart->get_cart() as $cart_item) {
        if (isset($cart_item['new_price']))
            $cart_item['data']->set_price((float) $cart_item['new_price']);
    }
}

// Front: Display option in cart item
add_filter('woocommerce_get_item_data', 'display_custom_item_data', 10, 2);

function display_custom_item_data($cart_item_data, $cart_item) {
    if (isset($cart_item['repair_price'])) {
        $cart_item_data[] = array(
            'name' => __("Repair option", "woocommerce"),
            'value' => strip_tags( '+ ' . wc_price( wc_get_price_to_display( $cart_item['data'], array('price' => $cart_item['repair_price'] ) ) ) )
        );
    }

    return $cart_item_data;
}

Code goes in function.php file of your active child theme (or active theme). tested and works.

Addition based on this answer: Display product optional cost in Woocommerce in order details

Allow to save the repair price as order item meta data and display it everywhere on orders and email notifications:

// Save chosen seats to each order item as custom meta data and display order items Warrenty everywhere
add_action('woocommerce_checkout_create_order_line_item', 'save_order_item_product_waranty', 10, 4 );
function save_order_item_product_waranty( $item, $cart_item_key, $values, $order ) {
    if( isset($values['repair_price']) && $values['repair_price'] > 0 ) {
        $key = __("Repair option", "woocommerce");
        $value = strip_tags( '+ '. wc_price( wc_get_price_to_display( $values['data'], array('price' => $values['repair_price']) ) ) );
        $item->update_meta_data( $key, $value );
    }
}

Code goes in function.php file of your active child theme (or active theme). tested and works.