"There are also times when it’s a good idea to make a spreadsheet, but even this can be overkill."
I think the point was not on the specific class but thinking of business from a macro perspective. The author was better able to grasp things and use his programming background to his advantage.
For the purposes of my example assignment, I am required to provide all answers rounded to the nearest dollar (hence the call to round().) For a one-off (or ten-off) series of calculations the use of floating point is a non-issue.
Time spent writing obvious reply: $2.50
Good faith deduction: ($1.00)
Deduction for joke comment opportunity: ($1.50)
Dear jbl, your invoice total is $0.00000027.
