Change fields in Zoho Invoice - HELP URGENT!

Change fields in Zoho Invoice - HELP URGENT!

Hi. This is urgent. I need to include a field on our invoice that states our GST number. I thought of replacing the PO field with GST, so did so when I edited our default template, but when I create a new invoice that field still remains PO#. Also, when I try and add a custom field, it works but when I export to pdf it says 'I need to change the pdf template.."

This is very urgent as by law we need this on our invoices in New Zealand.

I have attached a copy of an example of what the invoice currently looks like for your reference.

Please help asap!