Tax added twice

I’m using this plugin on the latest WooCommerce and WordPress versions.
The trouble is that the fee is being calculated with the VAT on it, and then if the tick box for “Taxable” is selected it’s then adding the value to the VAT sub total.
So the settings are: 15% Cart percentage. Taxable is ticked and no minimum order.
When a ￡495 product is put in the cart, the 15% fee shows as ￡89.10 (should be ￡74.25). The VAT shows as ￡116.82 (correct).

If I change the settings to: 15% Cart percentage. Taxable is unticked and no minimum order.
When a ￡495 product is put in the cart, the 15% fee shows as ￡89.10 (should be ￡74.25). The VAT shows as ￡99.00 (correct).

If I change the country to one that isn’t charged any VAT the figures are all correct!

https://www.ads-software.com/plugins/woocommerce-extra-fee-option/