for example:

subtotal - 79.38
PST 5.56
GST 3.97
total 88.91

84.93 is expensed to 'sundry' (must be manually totaled)
3.97 to GST

A possible fix could be:
Adagio had a field to enter the invoice total 88.91, then a field for the gst 3.97 it could then show a subtotal of the 84.93 and ask for a code and the data entry person wouldn't have to do any additional calculations!