If you need to add a suffix to the add to cart quantity input field at single product in WooCommerce then you can do it easily to using below code snippet.
Where add custom code:
You can add those code to your child theme functions.php or you can use code snippet plugin like this( https://wordpress.org/plugins/code-snippets/ ).
After adding this code snippet to your site you will see that a suffix will be added after the quantity input field at the single product page. Please check below screenshot.
Suffix added on single product page
So, as per above screenshot we see that, Liters suffix has been added after quantity input field on the single product page.
That’s All.
