Calculation error

love the plugin, it is great.
but I got one problem, it is the feature #3 mentioned in your description. It doesn’t work when I enter a calculation formula. Please help. Thanks.

3. Seller can define their own custom charge:-
The field via backend can accept calculation value, example. (4 + 4) / (6*4)

https://www.ads-software.com/plugins/add-tip-woocommerce/