Hi,
What he is saying is really strange.
The PayPal server should not see the name of the form sending the data.
Plus, the target=hss_iframe comes from the iframe mode.
So it seems that you still have the payment method configured with the iframe mode.
The file you want to look at is plugins/hikashoppayment/paypalintegralevolution/paypalintegralevolution_end.php
There, you can see a if at the beginning to decide whether to use the iframe mode or the non iframe mode.
So what you could do is to change that line from:
if (($this->payment_params->iframe == 1)) {
to:
to force the non-iframe mode and it should hopefully work fine.
In that file you'll also see the code
name="hikashop_paypalintegralevolution_form"
twice. So if you want to try and follow their recommendation, you can change both of them.
However, I don't see how that would work since javascript code there bases itself on the name of the form:
document.hikashop_paypalintegralevolution_form.submit();
so that you don't have to click on a button to submit the data to PayPal.
If you change the name of the form, this javascript wouldn't work anymore and you would have to manually validate the form with a button or change also the javascript line to use the new name of the form.