When you choose to cancel your ShipStation subscription, a number of things happen. Your subscription will still be valid until the last day of your billing period, so you will still be able to use the app until that time.
After your subscription ends, you will still be able to login to the app if you want to re-subscribe, but you will not be able to access any other portion of the app. At time of cancellation, we will no longer provide you with a free Stamps.com USPS postage account. Also, since you won't have access to the interface outside of the My Subscription page, run any reports on your shipment information for your own records while the subscription is still active.
If you have remaining funds in your Stamps.com postage balance, please contact them directly to ask about refunds.