Coupon Check Out Calculation Problem

Hi, Seyi,
This plugin works excellent.
I just found one problem, don't know how to configure it.
I use Joomla 2.5.14 + VM 2.0.22a + OPC + Latest AWO. I don't know how to attach a screenshot, you can find it here: http://www.deontewines.com/screenshot-ticket-1.jpg
In the check out, the product subtotal is $90 (10% GST included). And after the 5% discount coupon, the GST is still $8.16(=$90/11), it should be $7.77(=$85.5/11).
The GST in VM is set to be VAT TAX PER PRODUCT.
In AWO Configuration, I've set the "Calculate the discount before tax (Coupons)" to YES.
What I need is to remain all the other the same in check out, but change the GST to $7.77. Thanks.

Hellio,

I am not able to reproduce this problem. Can you try disabling opc for testing and trying it again?

Hi, Seyi,
I disabled OPC. And the screenshot is: http://www.deontewines.com/ticket-screenshot2.jpg. The tax is correct.
Is there any configuration I should do for OPC?

Ok, is your coupon processed through ajax in OPC or is the page reloaded. If it is through ajax, try disabling the ajax and try it again. If that does not work, then I really do not know. I would suggest contacting the opc developer.

Hi, Seyi,
Your answer is very helpful. I eventually solved this problem. I am not sure if it is the correct solution. Just share here maybe it could help somebody else.
I found it is related to the payment methods settings. In Virtuemart backend, go to shop>payment methods, in the payment method setting, choose configuration. In the TAX, if I choose "default rule", it will show the Wrong GST. If I choose "Apply no rule", everything is OK.
And thanks for your help again. Your support and your product is great.