Fault with Tax Total field on customised invoice with GST S15 & Exempt items

Be aware:

We have some customers who pay by credit card.  We charge them the credit card fee.  As a finance fee this is exempt from GST. On our customised invoice we have included the field [Tax Total].

This field is correct on our printed invoice until we add a small exempt credit card charge to the invoice.  When we add this exempt item to the invoice although the invoice total and gst are correct on the screen when it prints to our invoice it states [Tax total] as nil.  The grand total is correct.

To fix this I have changed the form and no longer use the [Tax total] field.