If you are wanting to see your current postage balance within ShipStation you can find this under Settings>Shipping> Carriers & Fulfillment. You will see a balance displayed in green right next to the add funds link.
If you want to see transactions, history, and credits applied you would go the following:
Go to Settings>Shipping> Carriers & fulfillment, click Settings on the row of the stamps account you want to view and click reports from the pop-up settings menu.
You will be taken to a stamps reports page and will be able to view a few different reports about balances, expenses, and prints. You will want to stay on the balances page and make sure you change the balance report view from Trend to Transaction. You can even export the results by clicking on the "Export" text in the upper right hand as shown in the gif below.
READING THE BALANCE TRANSACTION REPORT
Credit - this event will show up when stamps adds a credit to your account (refunds)
Print - This shows events where you created a label but used available postage balance
Purchase - This will show up is when you add funds to your account balance.
Adjustment - this will show as a charge if you had to purchase additional postage to get your package to its destination