There is not a way to split payments between two payment methods at DesignBundles or FontBundles.
To use your Store Credit balance you will need to ensure your cart is at or below your available credit amount, add more store credit, or pay in full - with a different method.
Any unused store credit will be converted into Plus Credits after 90 days. You will be notified with 1 month remaining and then again at 1 week. This conversion rate will be $1- 1 credit, and these credits can be used in our Plus product store.