Upgraded to 1.5.7 c, PHP Ver 7.4 and everything good until the order process throws a 500 error at last step of order confirmation. Error log shows this among other PHP warnings that follow in log:

PHP Warning: A non-numeric value encountered in *******/includes/classes/order.php on line 515.

Here is code from order.php:

Code:
    // Update the final total to include tax if not already tax-inc
   if (DISPLAY_PRICE_WITH_TAX == 'true') {
     $this->info['total'] = $this->info['subtotal'] + $this->info['shipping_cost'];
   } else {
    $this->info['total'] = $this->info['subtotal'] + $this->info['tax'] + $this->info['shipping_cost']; //THIS IS LINE 515
    }
I made other customizations to the cart so I am not sure where I went off the rails here. Tax and shipping are not part of price - they are calculated by site owner after order processing. I wonder where to look for problem. Any ideas?