Hi,

I'm having a problem with tax calculations when discount coupons are used.

The problems only occur when the store setting "Display Prices with Tax" is true.

There is a problem with the way discount coupons interact with tax and with shipping. Let's assume we're applying a 10% discount coupon.

The trouble is the discount coupon code uses the displayed total price in it's calculations. And these calculations assume that the total does not already include tax. If you have the discount coupon setting include_tax set to false the calculation will include tax anyway (approximately) and if you have it set to true then then the tax is applied a second time.

With include_tax set to false the calculations are close to what they should be if include_tax was set to true, but not spot on. The VAT is always out by 10% of the VAT of the shipping.

Any help on this would be much appreciated.

Fergus