It's been a long time since I was in that end of the business, but I do remember getting discounted fees (including bounce rates) because of our volume - this was for a boutique subsidiary of a national chain. At that time (more than 10 years ago), normal bounce fees were around $15 but we paid $5.
However, when I was an office manager a short time later, we got charged the same thing for bounces as the average consumer. I'd imagine it depends on the bank and on the size of the customer/company.