Summary
Excess money in an account can be applied to future Invoices as credits.
Key Point
At Settings > Invoices, select Automatically apply positive credit balance
Click the Setting gear icon in the upper right
Click Invoices
If it is not selected, click [X] Automatically apply positive credit balance
Click Save
Note Updating this setting does not implement the change in existing invoices, only invoices generated after clicking Save in step 5.