If you run into an "Over Balance" message in the app, this is likely due to a recent change to your balance that has resulted in your current outstanding order total being greater than your current available balance.
Please see our article on "Why has my balance changed?" if you haven't already to find out why there was a change to your balance.
Paying down your existing installments will reduce your outstanding order total. Once your outstanding order total is less than or within your new balance, you should be able to continue using Zip.