Xero's system operates to 2 decimal places. A lot of other systems operate to 4.
Let's use the example below of a bill from a New Zealand company to a New Zealand company, where GST is applied at 15%.
The Supplier's bill might say
Row 1, Tomato Sauce, $10.50
Row 2, BBQ Sauce, $10.50
Bill total $24.15
But, when we send data into Xero, Xero calculates the GST on each row, then rounds that GST up or down to 2 decimal places. So Xero would determine the GST on each row to be $10.50, times 15%, = $1.575, which it would then round up to $1.58 per row. Therefore Xero would determine the GST on this bill to be $3.16, not $3.15.
To compensate for this, invitbox will send into Xero an extra rounding row, in this occasion of -$0.01 (not taxed). To this GL Code we will assign the GL Code that was assigned to the last row of the bill.
Note, we know that this will not fix the GST amount, but it will balance the total of the bill, meaning you will be paying the supplier the correct amount of money.
What if you are NOT getting a rounding row in Xero?
Then check that you have a rounding GL Code and it's in Invitbox and also make sure it's ticked (meaning it's in use)
This can be checked via the Admin tab> GL Codes
UPDATE: As you may be aware Xero now operates using 4 decimal places. We will be updating our system in the near future to coincide with this.