Pay Advantage calculates the relevant GST based on each individual fee and not the total invoice amount. As some of these fees can be quite small this can produce a rounding anomaly which may mean the GST on your invoice is not exactly 10%.
The rounding of the GST per fee is necessary as a debit can fail or need to be refunded and we need to be able to handle the individual fee amount rounded to two decimal places.
If your agreed direct debit rate is $0.95 per debit the fee and GST portion will be rounded to 2 decimal places and calculated as $0.86 + $0.09 GST. In this instance, if GST is applied at 10% the GST amount would be $0.086 which is then rounded up to $0.09. Although the amount is small, when multiplied by large transaction volumes this can lead to the GST being slightly more (or less) than if you simply calculated the GST on the total invoice amount.