I do the same as Tom. My discounts are based on my ability and and willingness to manage change. We are not working a high volume sales environment with cash drawer and help ... we're handling between 10 and 50 or so product that must be managed, usually personally and with the focus in the chat with the customer/listener.
YES - Seamus I've had venues charge a percentage. I'd rather they charge a set fee, but some do a percentage - and that hurts. I make sure my base is high enough to compensate. I understand they need to cover their expenses, but I have to figure how much it will cost me and be sure the whole package is worthwhile.