Coupon with percentage discount and VAT

[ZeroPointLabs]

VirtueMart 3.0.8, Joomla 3.4.1

I've got the following order

Item	Ex Tax Price	Tax	Row Total
Product A	500 EUR	115 EUR	615 EUR
Product B	500 EUR	115 EUR	615 EUR
Subtotal	1000 EUR	230 EUR	1230 EUR
Shipping	100 EUR	23 EUR	123 EUR
Total	1100 EUR	253 EUR	1353 EUR

All is good and this is supported by the following variable values

$this->cart->cartPrices['basePrice']	1000
$this->cart->cartPrices['taxAmount']	230
$this->cart->cartPrices['salesPrice']	1230
$this->cart->cartPrices['shipmentValue']	100
$this->cart->cartPrices['shipmentTax']	23
$this->cart->cartPrices['salesPriceShipment']	123
$this->cart->cartPrices['billTaxAmount']	253
$this->cart->cartPrices['billtotal']	1353

I have the following coupon defined, which should give the shopper 10% off their order

Coupon	MYCOUPON
Value	10.00000
Percentage or Total	Percent
Coupon Type	Permanent Coupon
Minimum total Order	0.00000
Coupon Start Date	- Never -
Coupon Expiry Date	some time in future

when I apply the coupon to the order I would expect this (My testing shows that the discount is a percentage of the subtotal, so doesn't include the shipping costs)

Item	Ex Tax Price	Tax	Row Total
Product A	500 EUR	115 EUR	615 EUR
Product B	500 EUR	115 EUR	615 EUR
Subtotal	1000 EUR	230 EUR	1230 EUR
Shipping	100 EUR	23 EUR	123 EUR
Coupon	-100 EUR	-23 EUR	-123 EUR
Total	1000 EUR	230 EUR	1230 EUR

When I look at the cart on screen and the variables that support this, it doesn't look quite right

$this->cart->cartPrices['basePrice']	1000
$this->cart->cartPrices['taxAmount']	230
$this->cart->cartPrices['salesPrice']	1230
$this->cart->cartPrices['shipmentValue']	100
$this->cart->cartPrices['shipmentTax']	23
$this->cart->cartPrices['salesPriceShipment']	123
$this->cart->cartPrices['couponTax']	0
$this->cart->cartPrices['salesPriceCoupon']	-123
$this->cart->cartPrices['billTaxAmount']	228.13
$this->cart->cartPrices['billtotal']	1230

The value of 228.13 for $this->cart->cartPrices['billTaxAmount'] was initially a bit puzzling, but it turns out that it is the tax based on a bill total of $1220. i.e. it looks like the discount might be applied twice, once as a percentage and once as an absolute value. Well at least for the purpose of calculating the billTaxAmount.