Discrepancy Between Quoted and Agreement Price

I've had the same issue, for 10 years discounts were as stated.  Then starting in Dec of 2022 some wizard accountant decided to deduct the gst off some of the discount lines they were authorized to deal with.  Hence a $20 discount became 19.05 and so on.  Not all discount lines are calculated like this which is even stranger.  It's like giving you $20 less .95 cents+gst.  This makes no sense and didn't occur before 2022.  It's an error that has never been corrected and no matter how articulate you are at trying to explain, is frustrating they don't understand or correct it.  Spent hours but never get the right person (ie accountant) to address it.  Hours on the phone and I just gave up. 🥱