tax_amount and base_tax_amount can differ when using single currency and multiple rates

[erikhansen]

Steps to reproduce issue

• Setup a $75 product in Magento
• Create a taxing jurisdiction San Diego in AvaTax which will result in a combined 8% rate: CA STATE TAX (6.5%), CA COUNTY TAX (1%), and CA SPECIAL TAX (0.5%)
• Place an order for that product. The order's base_tax_amount will be 6.01 (correct) and the tax_amount will be 6.00 (incorrect).
  • The reason that the base_tax_amount is 6.01 is because AvaTax calculates and rounds the tax for each of the taxing jurisdictions and them combines them:
    • $75 x 6.5% = 4.875, rounded to 4.88
    • $75 x 1% = 0.75
    • $75 x 0.5% = 0.375, rounded to 0.38
    • Total: 6.01

Explanation of issue

The reason that the tax_amount is incorrectly set to $6.00 is because of this section in \ClassyLlama\AvaTax\Framework\Interaction\TaxCalculation::getTaxDetailsItem. As you can see, this code is built to handle properly calculating rates for non-base currencies, but because it's being applied to the base currency, it's calculating $75 x 6% and is coming up with $6.00.

/**
 * Magento uses base rates for determining what to charge a customer, not the currency rate (i.e., the non-base
 * rate). Because of this, the base amounts are what is being sent to AvaTax for rate calculation. When we get
 * the base tax amounts back from AvaTax, we have to convert those to the current store's currency using the
 * \Magento\Framework\Pricing\PriceCurrencyInterface::convert() method. However if we simply convert the AvaTax
 * base tax amount * currency multiplier, we may run into issues due to rounding.
 *
 * For example, a $9.90 USD base price * a 6% tax rate equals a tax amount of $0.59 (.594 rounded). Assume the
 * current currency has a conversion rate of 2x. The price will display to the user as $19.80. There are two
 * ways we can calculate the tax amount:
 * 1. Multiply the tax amount received back from AvaTax, which would be $1.18 ($0.59 * 2).
 * 2. Multiply using this formula (base price * currency rate) * tax rate) ((9.99 * 2) * .06)
 *    which would be $1.19 (1.188 rounded)
 *
 * The second approach is more accurate and is what we are doing here.
 */
if (!$useBaseCurrency) {
    /**
     * We could recalculate the amount using the same logic found in this class:
     * @see \ClassyLlama\AvaTax\Framework\Interaction\Line::convertTaxQuoteDetailsItemToData,
     * but using the taxable amount returned back from AvaTax is the only way to get an accurate amount as
     * some items sent to AvaTax may be tax exempt
     */
    $taxableAmount = (float)$taxLine->getTaxable();
    $amount = $this->priceCurrency->convert($taxableAmount, $scope);

    $tax = $amount * $taxLine->getRate();
    $tax = $this->calculationTool->round($tax);
}