You can add your tax rate and select your region which will be listed in plans
Go to Menu -> Settings -> Tax & Coupons
You can check the image as shown below:
Click on "Add Tax" pop shows up as shown below
Select Region, choose Tax Type & Custom Label and now enter tax percentage & select Inclusive/Exclusive and click ADD TAX to save.
New Tax will be added in the list
Apply Tax Rate in Pay link
You can apply tax tax while creating plans which will be listed when you create pay link.
Go to Menu -> Pay link-> Create Pay link
When you create Pay link, plans you created will be listed according to currency. In that , you can select plans based on tax
Apply Tax Rate in Invoice
In the same way, You can also apply in Invoice also
Go to Menu -> Invoice-> Create Invoice
When you create Invoice, you can add multi item but it is applicable only for one off also you can select existing plan from dropdown. Plans you create will be listed and give you preview on the right-side based on tax or coupon code added .