Custom button appearing on all product pages in Woocommerce

I’m currently using this code with a hook to a specific product but the button seems to be appearing for all other products on my website as well.

add_action('woocommerce_after_add_to_cart_button','additional_single_product_button');
function additional_single_product_button() {
$product_ids = is_single(871);
echo "<br>";
echo "<br>";
echo '<div>';
echo '<a data-type="inline"  class="button alt" href="/contact-us/">Pay at Spa @ $218 nett</a>';
echo '</div>';}

Here’s my website: http://offers.elements.com.sg/

Please advise how I can just add this button for a specific product. Thank you.