A negative balance is very possible as Sku Grid runs as long as you have credits in your account.  Similar to an on/off switch. If you have 1 credit available, it's still going to check and reprice your items at your specified interval. Since credits are actually tallied up for the day's usage and officially deducted the next day, that is how a negative balance is technically possible. Then at that point, the credit balance would be 0 or negative and then Sku Grid would not longer process anything or run.