Add-on kits for math?

Hi, we are currently using the 5-7 level and I was just looking ahead to 6-8 and noticed that a lot of the materials in the math kit are things that are also in the 5-7 kit. For example, we don't need a new abacus every year or another set of attribute blocks, but I don't see an option to go "a la carte" on the components we do need. Have you considered offering add-on kits for people who already purchased the previous level? I know Right Start Math used to do this.