Use unit pricing in WooCommerce

functions.php

add_action( 'woocommerce_get_price_html', 'wc_ninja_customize_price_html', 2, 10 );
function wc_ninja_customize_price_html( $price, $product ) {
    // Enter your product's ID here
    $product_id = 87;

    if ( $product_id == $product->id ) {
        $price .= ' ($2.00 per piece)';
    }

    return $price;
}