Let's go back to reporting bugs here.
The PayPal Standard has had a bug since Opencart 1.5.x, as follows:
Customer adds a taxable product to shopping cart, goes to checkout, and chooses Paypal for the payment method. The Paypal Standard payment method then uses some hidden fields like below in its payment form:
<input type="hidden" value="Shipping, Handling, Discounts & Taxes" name="item_name_
n" />
<input type="hidden" value="" name="item_number_
n" />
<input type="hidden" value="
x.xx" name="amount_
n" />
<input type="hidden" value="
x" name="quantity_
n" />
<input type="hidden" value="
x" name="weight_
n" />
Once the user is on the Paypal page and has logged into his Paypal account, Paypal applies the taxes again!
The taxes, discount, and handling values should always be submitted in hidden form fields like this:
<input type="hidden" name="tax_cart" value="
x.xx" />
<input type="hidden" name="discount_amount_cart" value="
x.xx" />
<input type="hidden" name="handling_cart" value="
x.xx" />
And the shipping or order fee values could be submitted as dummy items:
<input type="hidden" name="item_name_
n" value="
x.xx" />
<input type="hidden" name="amount_
n" value="
x.xx" />
I have created a VQmod XML script for the bugfix (see attachment), was originally written for 1.5.2.1, but should work for later versions of opencart, too.