P+i will attempt to calculate the tax, where it is outlined in the invoice. There are instances where it is included in the invoice total without mentioning the specific amount.
For example, it may list the invoice total and say that it is inclusive of 10% GST.
In this case, the invoice may be entered incorrectly and need to be adjusted. For all the line items that have tax applied, this should be taken out of the unit price. The easiest way to do this is to divide the amount by 1.1.
For example, if the line item for "Beverages" is $168.54 inclusive of 10% GST and the tax amount has not been added in, you can divide this by 1.1, to get the 'Unit Price' of $153.22.
Then, if you enter "10" in the 'Tax (%)' field, it will calculate the tax to $15.32 and the 'Total (Inc. Tax)' of $168.54.
To save time, you can click the [Update Invoice] button, and the message there will tell you what the expected total tax will be.