No need to worry about tax calculation. Just choose the tax amount field and it will calculate the total tax amount with predefined percentage.
In this demo form, we have chosen 10% tax rate which will be added to your cart amount.
And also the user will get an invoice of full details on his mail address.
Get payment with tax (percent) :
Test using card number 4242 4242 4242 4242, any (future) expiration and any CVC. More test cards