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I'm sorry for the confusion and inconvenience.
The Endicia accounts are prepaid postage accounts so they do not allow your balance to go down to $0.00. You can see this mentioned in the following article: Why can't I buy labels with an exact amount in my account?
I would recommend adding just a little bit more to your postage purchases so you do not get down so low that Endicia requires an additional purchase since they are prepaid accounts.