Need real help – Double TAX paid by customer

Please I need to sort this out as I am fed up writing to customers explaining why they were charged with double TAX. And it does not look professional at all!

When an order is made on the check out page before payment all is ok
subtotal + TAX =Total
83.70 +16.74 =100.44
(see shoping cart.png)

in order manager the total is just 83.70 without TAX
(see order-manager.png)

Order items shows price and TAX
(see order.png)

If I look at the invoice the TAX is there under the sub-total but it is NOT added so the total is just the sub-total!!
(see invoice.png)

The payment was made with Paypal and the TAX was doubled to 33.48 so the buyer paid a total of 117.18

Subtotal £83.70 GBP
Tax £33.48 GBP
Total £117.18 GBP
Payment £117.18 GBP

So, if I understand well. TAX is not added in Shopperpress but is taken 2 times from the buyer.

The paypal account is ok as it is used with another site which uses opencart and there is no problem with payments.

PLEASE HELP