There are no tolls on I-81 in PA or MD either. If you're taking the PA Turnpike to reach I-81, they take E-ZPass. Below are the Carlisle toll booths if you get off the turnpike at Exit 226, Carlisle, to connect with I-81.
After exiting the toll booths, you need to be in the left lane and exit onto US-11 north, Harrisburg, to reach the I-81 interchange.
Once on US-11 north, it's about a mile between the turnpike and I-81 interchange. There are truck stops/filling stations/restaurants/etc. along this stretch. If you have the need for any of these services, it's a good place time to stop as many of them are "big rig" friendly. The entrance ramp for I-81 south, Chambersburg, will be on your right and look like this: