You can configure ShipEngine to automatically purchase postage when your Stamps.com account balance dips below a certain amount.
Click the Carrier Management. From your Current Carriers list, click Add Funds for your Stamps.com account.
To configure your auto-purchase settings, click the button next to Set up Auto-Funding. From here, you can set the minimum balance before more postage is automatically purchased.
For example, if you wanted to purchase $100 by default any time your account balance reaches below $250, set the When balance drops below amount to "250" and the Automatically fund by this amount to "100". Then enter in the maximum amount of times ShipEngine should automatically purchase postage in a day next to Stop after this many times per day.