Tax calculation errors when more than one discount coupon applied

Just a quick update, I've played around with the line item weight in Store -> Configuration -> Coupon module settings and I can change the order of the discount coupon in the summary, but it does not affect the tax calculation. I will therefore withdraw my earlier speculation, leaving me clueless for now...

Hi,

I've managed to track this problem down, not sure if it should be classified as a uc_coupon bug or a uc_taxes bug, but this is the cause and my hack solution.

The problem is that uc_taxes_apply_tax in uc_taxes calls the uc_coupon_tax_adjustment function in uc_coupon for EACH coupon line item in the order, but uc_coupon_tax_adjustment processes ALL coupons each time it is called, so if you have 2 coupons applied it will take the discounts off the taxable amount twice.

To fix it I just modified the uc_taxes_apply_tax function so that it only calls the adjustment once, so this:

  if (is_array($order->line_items) && is_array($taxed_line_items)) {
    foreach ($order->line_items as $key => $line_item) {
      if ($line_item['type'] == 'tax') {
        // Don't tax old taxes.
        continue;
      }
      if (in_array($line_item['type'], $taxed_line_items)) {
        $callback = _uc_line_item_data($line_item['type'], 'tax_adjustment');
        if (isset($callback) && function_exists($callback)) {
          $taxable_amount += $callback($line_item['amount'], $order, $tax);
        }
        else {
          $taxable_amount += $line_item['amount'];
        }
      }
    }
  }

became:

  $coupons_processed = false;
  if (is_array($order->line_items) && is_array($taxed_line_items)) {
    foreach ($order->line_items as $key => $line_item) {
      if ($line_item['type'] == 'tax') {
        // Don't tax old taxes.
        continue;
      }
      if (in_array($line_item['type'], $taxed_line_items)) {
        if ($line_item['type']=='coupon' && $coupons_processed) {
          // don't process coupons again
        } else {
          $callback = _uc_line_item_data($line_item['type'], 'tax_adjustment');
          if (isset($callback) && function_exists($callback)) {
            $taxable_amount += $callback($line_item['amount'], $order, $tax);
          }
          else {
            $taxable_amount += $line_item['amount'];
          }
          if ($line_item['type']=='coupon') $coupons_processed = true;
        }
      }
    }
  }

That's in uc_taxes (modules/ubercart/uc_taxes/uc_taxes.module) at about line 600.

Regards
Hans