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Unfortunatelly, Tax is not an included option in the Payment tools of Jotform. So, you cannot add calculation of tax payment to the products amount.
A workaround you can do is, to add Tax amount as another item and make it a required and selected as default. This way, the payment form will always have Tax amount on it seen by users.
Refer to this guide also: -How-To-Add-Options-In-The-Payment-Form
(look for section V. Add Mandatory Payment and Default Product )
Or you can just always priced the items with inclusive tax -- then, just add annotation that your items/products are already tax inclusive.
For complex method, review this thread, it requires using the form's source code and you must embed it on your own website.
Hope this help. Inform us if there is more we can help you with.