It's likely that you're using a plan that is too small for the amount of items you have in the account and usage based on your settings. So your current plan will not last you until it renews again.  Sku Grid does not automatically charge you when credits are low.  You have a few options.

1. Add One Time Purchase Credits to get you through until your subscription renews.

2. Cancel the Current Preapproved Payment Agreement you have in your paypal account and then add a new subscription (basically, that's like starting over with your credits early)

3. Add Another Subscription and still keep your current.  If you want to do this, you'd keep your current subscription and just add another.  You'd be running more than 1 recurring subscription until you choose to cancel one.

Some helpful videos are below.
1. How to Estimate Credit Usage and Choose the Right Plan -
2. How to Cancel Your Sku Grid Subscription -
3. How to Add a Subscription -