pr: [Nightly Fix] - Bug - Fix Double Tax Amount in Coupon Discount Calculation

[jewel-claw]

Summary

Bug: Tax is double-counted in the transaction record when a coupon is applied.

Root cause

In SubmissionHandler::handleSubmission(), $paymentTotal is built by summing all line items including tax line items:

foreach ($paymentItems as $paymentItem) {
    $paymentTotal += $paymentItem['line_total'];
    if ($paymentItem['type'] == 'tax_line') {
        $taxTotal += $paymentItem['line_total'];
    }
}

So $paymentTotal = subtotal + tax.

When a coupon was applied, the old formula was:

$paymentTotal = ($paymentTotal - ($paymentTotal * $discountPercent)/100) + $taxTotal;

This re-adds $taxTotal after applying the discount — double-counting tax in the stored transaction.

Example: $100 subtotal, $10 tax, 10% coupon

• $paymentTotal before = $110
• After discount: $110 - $11 = $99 ✅
• Old code: $99 + $10 (taxTotal) = $109 ❌ (overcharge)

The original developer even commented //issue on bottom line flagging the problem.

Fix: Remove the erroneous + $taxTotal. Tax is already included in $paymentTotal and is correctly discounted proportionally.