At the moment, Paythen doesn't support showing or adding tax separately as line items on the payment page. Any payment amounts in Paythen plans should include any applicable taxes.
In your Stripe account though, if you are using Stripe tax, it will work as expected and automatically calculate any taxes you owe based on your and the customer's location. Paythen collects the country (and where required, a zip/post code) which allows Stripe to calculate taxes.
At the moment, a breakdown of taxes is not visible on the front-end for customers on any plan payment pages. The only way to show this is to explain this (and list any taxed amounts or rates) in the plan description for customers.
We are working on adding support for tax across our supported countries. This includes manually added tax rates based on certain conditions as well as the new Stripe Tax feature which automatically calculates tax based on your customer's location.