GST error

David.  You really haven't given a lot of information here regarding just what this error is.  However, I can assume that it relates to tax rounding which manifests itself when you have a lot of items in a particular invoice.  Here is an example.  Let us say that you have 10 different items of just 6 cents before GST.  One tenth of that amount is 0.6 cents but of course you cannot invoice anybody for just 0.6 of a cent, but it has to be 1 cent due to rounding.  A total of 10 of these transactions  would have a value of 60 cents which will have accumulated GST of 10 cents (10 x 1 cent) but when you look at the total of the invoice, the immediate reaction is that the total should only be 60 cents + GST of 6 cents = $0.66), but the invoice in fact says 10 x 7 cents (including GST) which is $0.70.  I do not believe that any program is capable of randomly changing the value of items so that the GST amount works out exactly.  I have seen plenty of examples of this where an invoice contains multiple items, where the overall total of the GST is not exact, and I recognise this but ignore it.  Obviously you have pedantic customers who don't like what they see.  One would expect that in the basis of averages, items accumulating too much GST would be offset by items from the opposite point of view.
There is however a solution David, which you can address yourself.  It is very simple.  Make sure that the cost of every item, before tax, ends with a zero at the end of the cents and your invoices will then be perfect. 
There is at least another option that I am aware of which would require you to have the GST price of your item be reflected as say to 3 or more decimal points of a cent and that should also fix the problem.  This is not a flaw with Reckon itself.
John L G