If they're sales people and bean counters, then they will definitely understand the almighty dollar (pound, euro, etc.). Can you demonstrate that the time spent to maintain these extra columns doesn't justify the added sales?
Be very careful here and make sure your argument makes sense. I've found myself resistant in the past to doing customizations more because I didn't want to ugly up my pretty little domain model than because it would really be that difficult to maintain. A decent analysis will help you determine why you're resisting the customization.
Remember - the bottom line is that you need to keep clients happy in order to stay in business. We thoughtful developers can sometimes lose sight of that in our quest to make things maintainable and simple.
