> I looked at the software, it had two separate copies of the invoice calculation (separate for on screen and for printing to PDF).
That's actually kinda normal, in some industries.
For example, an amazon marketplace seller's warehouse management system might not be tightly integrated with Amazon's basket/checkout display logic.
In some situations the results of recalculating are supposed to be different. For example, if there's a "5% off when you buy 3 widgets" offer and you check out with 2 widgets in your basket, the offer doesn't apply. But if you checked out 3 widgets, thus getting the offer, then the seller found they were low on stock and could only send you 2, you should get the 5% discount on those 2.
That's actually kinda normal, in some industries.
For example, an amazon marketplace seller's warehouse management system might not be tightly integrated with Amazon's basket/checkout display logic.
In some situations the results of recalculating are supposed to be different. For example, if there's a "5% off when you buy 3 widgets" offer and you check out with 2 widgets in your basket, the offer doesn't apply. But if you checked out 3 widgets, thus getting the offer, then the seller found they were low on stock and could only send you 2, you should get the 5% discount on those 2.