How does FASTag calculate charges for round trip?

I have been a regular user of FastTag from 2017 onwards, and I must say, it's very convenient at the Toll Booths. Especially during the initial days, when there were very few users, it used to a Royal Treatment and one would get out of those crowded toll booths in no time.

Now, that it's made mandatory, I need to clarify and document a few observations which I have encountered during this short stint.

1. I had bought the FastTag from Paytm using their online Portal. They charge around 350/- INR for the RFID Sticker and they would keep 150/- INR in your FastTag wallet of the Paytm app. That acts as a reservation for your next toll booth and if it goes below that, it will ask you to recharge your Paytm regular wallet.
2. At run time, each transaction will be actually consumed from the Paytm Wallet, and the reserved wallet will always be kept at 150/-. 
3. When you cross a toll booth, there will be a scanner above, which will scan your windshield and automatically deduct the amount from the reserved amount and approve your payment. The boom barrier opens when the transaction is a success
4. Many times, there could be issues, and most likely than not will need to move your vehicle back and forth to get closer to the Scanner. 
5. When the scanner on top, doesn't seem to help, they will do the scanning using a Hand-Held Gun Scanner.  From my experience, this always works.
6. The return journey always costs lesser and to take care of that, fast tag, charges the One-Way charge when one crosses a given toll booth. If one returns the same toll booth with 24 hours of the 1st transaction, the 2nd one will only charge the difference of the Two Way charge and the One Way charge.
1. Example - If the one-way charge is 90 Rs and Return journey is 135 Rs, then while going the first time, it will charge 90 Rs, and while returning will charge 135-90=45 Rs only.
7. One does get an SMS with the amount charges and the time when you had crossed the toll booth. But normally I have seen a delay in receiving the SMS.

If you have any queries, then please ask in the comment section.