Tax calculations wrong on Discount Coupons

We're based in Ontario, and after following the Canadian Tax setup instructions, tax calculations all seemed to be working well, until I tried introducing a discount coupon.

Here's an example:

Sub-Total: CAD$165.00

Discount Coupon: xxxxxx : -CAD$16.50

Canada Post: CAD$10.05

5.0% GST#84583 6626 RT0001 + 8.0% PST (Ontario): CAD$19.31

5.0% GST on shipping + 8.0% PST (shipping): CAD$1.31

Total: CAD$181.31

Total should be $179.17. The difference, an extra $2.14, appears to come about from calculating the tax on the undiscounted price ($165), which would produce a tax of $21.45 ($21.45 - $19.31 = $2.14).

Happens whether I make the discount -10% or -$16.50. Also happens whether order total sort order puts the discount right after the subtotal (as above) or in the default (280) order.

Current discount coupon module settings are...

Include shipping: false
Include tax: false
Re-calculate tax: credit note
Tax Class: GST/HST/PST Class

If, instead, I set recalculate tax to "standard," the problem seems to go away but is replaced by another one (which is why I switched to "credit note" in the first place): Tax on shipping is miscalculated as $1.18 (should be $1.31), as if the shipping charge was also discounted by 10% (even though it does not appear as discounted in the order total, but rather at the full $10.05). Total in this case is listed as $179.03, which is also wrong (even if the shipping tax was supposed to be discounted) as the actual sum of the listed amounts is a penny more, $179.04.

Finally, if I set recalculate tax to "none," there are elements of both problems above: tax on shipping is listed as as $1.18 (should be $1.31) as when recalculate tax = standard, tax on product is listed correctly but order total ($181.31) is apparently based on tax on undiscounted product ($2.14 too high) as when recalculate tax = credit note.

I'd be grateful for some advice. I'm running the current version of Zen Cart.

Thanks,
Derek

Hi Derek,

I have the same kind of problem :(. Have you found a solution yet?
Thanks.

Luna

Sorry for the slow reply. In the end, for various reasons unrelated to this technical problem, we decided not to offer discount coupons as part of our marketing strategy.

But I never did find a solution to the problem. I came across another post on this forum that described what seemed to be an identical problem, and it also apparently hadn't been solved.

Sorry dkoehler - haven't fully read your post, but from the first few lines what comes to mind is the same problem we had and after 12 months finally figured it out.

Go to "Modules" - then "Order Total"

In there it sets up in sort order what happens when in calculation.

Problem we had was that we loaded coupons including sales tax (GST) and then was taking the amount off the total before sales tax was applied.

Hope that helps and sorts the problem for you.