For any commission type (percent of sales, flat rate per order or flat rate per item), you can either select the simple rule or the advanced rule. The advanced rule allows you to assign different values of commission rate to different levels of order value.
To set up the advanced commission structure, go to Programs > Edit Program > General > Commission Structure. You can also set it up when creating a new program (Programs > Add program).
For the rule, select Advanced:
From there, you can set a rate for the level 1 orders (orders with the value of $0 or above).
Next, add more level(s) of commission rate by clicking Add new level. From there, you can set the minimum order value and commission rate for each level.
For example, if you want to give your affiliates 10% of sales for the under $100 orders, and 15% of sales for $100 or above orders, you can set up the commission structure like this:
Don't forget to click Save (or Add if you're creating a new program) to finish setting up the program.