Flockel,
This is the reply I got from them eventually:
Hello and thank you for contacting NCH Software. We apologize for the inconvenience this may have cause but what the program is doing is that it rounds up the 0.55 to 0.60 and that is why it shows 6.76 as the GST instead of 6.75. If the total price of the item is 67.50 then the GST will be applied as 6.75.
I have reported this issue to our developers and they will check on this and determine if that can be changed and will notify you by e-mail at the moment there is an update on this case.
Phil