Help with Simple Quantity Discount

[Guest]

Am hoping somone has done this before I go out and re-invent the wheel.

 

I need to do a simple quanitity discount. The basic need is to be able to provide a 5% discount if the total quantity for all products purchased in a certain category exeeds 100.

 

For example, we offer 6 products linked to categories_id 21. If a customer orders a total of 100 pieces, in any combination of the 6 products, I want to give them a 5% discount and note as such on the checkout page.