You can configure ShipStation to automatically purchase postage when your ShipStation Carrier Services account balance dips below a certain amount.
Click Settings (the Wrench icon in the upper right).
Next, select Shipping and then Carriers & Fulfillment from the sidebar on the left.
From here, select the Add Funds link for your ShipStation Carrier Services account to bring up a new window with the option to purchase postage Manually or Automatically.
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 purchase this amount to "100". Then enter in the maximum amount of times ShipStation should automatically purchase postage in a day next to Stop after this many times per day.