Unfortunately, I still get the error when both are "fixed"
Code:
      $this->responseText['priceTotal'] .= number_format($this->shoppingCart->total, $decimal_places, $decimal_point, $thousands_point);      $this->responseText['preDiscPriceTotal'] = number_format(zen_add_tax($this->shoppingCart->show_total_before_discounts, zen_get_tax_rate($product_check->fields['products_tax_class_id'])), $decimal_places, $decimal_point, $thousands_point);
    } else {
      $this->responseText['priceTotal'] .= $currencies->display_price($this->shoppingCart->total, 0 /* zen_get_tax_rate($product_check->fields['products_tax_class_id']) *//* 0 */ /* DISPLAY_PRICE_WITH_TAX */);
      $this->responseText['preDiscPriceTotal'] = $currencies->display_price($this->shoppingCart->show_total_before_discounts, zen_get_tax_rate($product_check->fields['products_tax_class_id']));
    }
One checker advised to change
Code:
protected string $responseType = 'success';
to
Code:
protected $responseType = 'success';
for php 8 compliance. That had no effect.

One checker also suggested that the $_post variables are not properly set before being used.