Discount on total amount of sales

The user has asked to able to give a discount once a certain dollar amount of sales have been reached. Once that amount is reached, the discount will be given at the time of sales order entry and the rate of discount would be different depending on the item.

For example - once a customer reaches a total of $10,000 in sales, they get 5% off of item type 1 and and 10% off of item type 2. These discounts need to occur at sales order entry.

I'm struggling with getting the accumulated sales part. Can anyone provide any input on how best to handle this?