"Notice: Undefined offset: 8 in /var/www/html/components/com_payplans/helpers/modifier.php on line 207"
I had a look at the file and both FIXED_NON_TAXABLE_TAX_ADJUSTABLE and PERCENT_OF_SUBTOTAL_NON_TAXABLE
had the same line: $total[8] = $total[8]+$result;
so for PERCENT_OF_SUBTOTAL_NON_TAXABLE I changed the line to $total[9] = $total[9]+$result and it works now.
Not sure how this happened. Either the core files were messed up or it was due to a Payplans plugin I installed.