Marketpress coupon tax miscalculation error

When applying a discount to a purchase the tax is calculated on the original price and not the discounted price. In essence over charging the tax rate. You can imagine the legal implications of this.

You can see this here
http://cutlerypolisher.com.au/store/products/cutlery-polisher-catersave-cs-3500/
use coupon code google16

tax is $630, after discount, which is incorrect, how do we fix this?