We noticed that as well, and we have actually brought that to Krowdpop/KPOP United’s attention. We’re not sure if they would do something about that anytime soon.
However, there is a way around the glitch. You can click on the actual item you would like to pre-order and then you can add it to your cart from the item page. You will have to do this for each item you want to pre-order. We found that this is the only way that works for now.
Let us know if you still have problems!