When you attempt to purchase labels with only the exact the amount available in your postage account, Endicia will not allow the purchase. This is because an Endicia postage account is a pre-paid account from which you buy postage. Your account cannot have a zero balance once the account is fully set up and funds have been added.
In order to complete the postage purchase, please add more postage to your Endicia account.
Or, you can set up automatic postage refills.