in reply to
Writing a Billing System
I'm actually in the middle of doing something fairly similiar to this so I know what you're going through. I can think of 2 possibile solutions off the top of my head.
A) If the action will always be a discount percentage you can just store the discount (15%). This will also help maintenance when the price goes up in 6 months.
B) If there are multiple types of actions you can store 2 fields. 1) reference to type of action (percentage, promo price etc) and 2) value of the action (15%, $9.99 etc). The user would then select a type and a value and be on their merry way.