Only on the Free plan, Undo restores your monthly limit. We anticipate that users will use it as a testing ground for Loader features at first. For our paid plans, all records imported by Loader to your app are considered successful. Failed records do not effect on your monthly limit.
There is also the option to stop the import in progress.
If you are unsure about any import or update setup on the paid plan, we recommend opening another free account with Loader and testing it there. You can link your app to multiple Loader accounts without any limitations.