Well I did had this code on functions.php for a woocommerce in wordpress.
Basically is a checkbox so when checked or unchecked it adds or doesnt the percentage fee in adition to the normal tax (is a kind of special tax in Spain).
But since like a year it no longer works and I don’t know why. I tried to change variables names or even delete plugins who may interact with fees but… it just doesn’t work and few years ago it worked. Or maybe is probably the code itself.