Unused daily credits are not carried over to the next day, so make sure you use all your credits for refreshes if you no longer need to post new listings.
Example: you own the 25 listings package, which entitles you to 5 daily credits. On Monday you post 5 new ads, which consume all of your 5 daily credits and you can no longer place new ads or refresh an ad that day. On Tuesday your 5 daily credits are regenerated and you can use them for example to refresh 2 of your ads and place an additional 3 ads. And so on.