Tax
You can add your region and tax rate and select the same while you Add Plan
Go to Menu -> Settings -> Tax & Coupons
You can check the image as shown below:

You can edit the Region, Type, Custom Label, Tax Rate and Status by clicking on the text of the already given regions.
You can click on the link for a video tutorial:
Click on "Add Tax" pop shows up as shown below:

Select Region, choose Tax Type & Custom Label and now enter tax percentage & select Inclusive/Exclusiveand 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 -> Paylink-> 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 as shown:

You can click on the link for a video tutorial:
Inclusive Tax:
Exclusive Tax:
Multiple Tax adding:
Apply Tax Rate in Invoice
In the same way, You can apply in Invoice also, Go to Menu -> Invoice-> Create Invoice
When you create Invoice, plans you created will be listed according to currency. In that , you can select plans based on tax as shown:

You can click on the link for a video tutorial: