How to make price option negative in WooCommerce Product Add-Ons plugin?

Question

WordPress

In the plugin, you can select options for the product, but they can only increase the price of the product, how to make them able to reduce the price of the product.
I looked at the source code, there are some restrictions on option['price'] in different .php files, the value must be greater than 0, but removing this I did not get the proper result, I also corrected the total > 0 line in addons.js, also nothing happened.