Percentage coupons appear to incorrectly calculate VAT depending on module config

Here's an example, question follows:

I have an item set up in our shop (1.3.9f) which I configured a 25% off discount coupon for where the minimum start price for the coupon is 5.98 - the total price when purchasing two of that item.

However, when testing it, this happens:

£4.78 = threshold for discount coupon becoming redeemable when tested
£5.98 = list price of 2x shop item, 2.99 each (no VAT)
£4.49 = the actual price of 2x item, with 25% discount

Zen cart is calculating that the discount of 25% = £1.20 on a £5.98 purchase => £4.78 ?!

£4.79 / £5.98 = 0.799 (80%), which means Zen Cart is only taking off 20%, not 25% as instructed.

Also, amusingly the coupon just seems to be being negated by the VAT of £1.20, which is the correct VAT for the shipping.

Sample checkout total:

£5.98 Sub-Total
£7.18 Courier - UKMail (UK & Ireland Next Working Day) (Shipping to GB (1 x 0.82kg(s)))
-£1.20 Discount Coupon: coupon25pc
£1.20 Zero-rated VAT for coffee, sweets etc. + VAT @ 20%
£11.96 Total

If the discount coupon shipping module is set to Include Tax, this happens:

£5.98 Sub-Total
£7.18 Courier - UKMail (UK & Ireland Next Working Day) (Shipping to GB (1 x 0.82kg(s)))
-£1.50 Discount Coupon: coupon25pc
£0.90 Zero-rated VAT for coffee, sweets etc. + VAT @ 20%
£11.66 Total

Confusingly, this second total is the correct amount, but I don't know where the 90p VAT is coming from (because the £7.18 courier cost already includes the £1.20 of VAT @ 20%) and usually Zen Cart's VAT total just reflects this.

HOWEVER, with the discount coupon shipping module set to Include Tax = yes, I can increase the Coupon Minimum Order to 5.98 (the correct amount) and the coupon functions correctly.

Any idea what's going on? I can't figure this one out. Running 1.3.9f, Ceon SEF URIs and Stock by Attributes but this product has no attributes. The shop item in question for this discount coupon has 0% VAT (it's coffee); all shipping modules are set to calculate VAT at the standard rate (20% for the UK).

Another example:

A customer checks out with two items totalling £12.98 = £6.49 each including vat (so for both, £2.16 VAT).

£8.28 royal mail delivery (this amount, as displayed in checkout, already includes VAT of £1.38)

discount coupon -£3.50

The checkout shows vat of £2.72
And a £17.76 total

is this correct? No! The final total is correct but everything else shows incorrectly. Here's what it should be:

£5.41 excl vat for one t-shirt = £1.08 VAT, so
£10.82 excl vat for two shirts = £2.16 VAT for two.

£8.28 for royal mail = £6.90 excl vat, £1.38 vat - this appears to never be displayed in the checkout price (although it may be because of the Royal Mail shipping methods, as obtained from the Zen Cart add-ons site and updated with the latest RM prices myself)

So, at the moment we have £2.16 + £1.38 = £3.54 total VAT.

If we MINUS 3.50 for the coupon BEFORE VAT is calculated on the T-shirts, you get £7.32 for t-shirts = £8.78 incl vat, £1.46 VAT.
If we MINUS 3.50 for the coupone AFTER VAT is calculated on the shirts, you get £7.90 for t-shirts = £9.48 incl vat = £1.58 VAT.

£1.46 + £1.38 = £2.84 VAT.
£1.58 + £1.38 = £2.96 VAT.

From this, I can only discern that not only does the displayed VAT on the checkout not include the calculated tax on the postage (unsure if this is ZC or the add-on at fault, but both the table rate AND Royal Mail shipping modules exhibit the same behaviour) - there's also a discrepancy showing the VAT in the checkout, even though the final total is correct! Unfortunately this is still a problem as we do have business customers who absolutely require a correctly-stated VAT statement for their own accounts, so I'm wondering the heck is going on. Any and all insight really appreciated at this point.

In the meantime next week, I'm going to set up a new vanilla store and try to get it all working... again... including showing correct tax. I cannot WAIT for ZC 1.4 :rolleyes: