I deleted a post where hardware used to pirate flight sims was discussed.
Let me help clarify how this works.
If you purchase RealFlight Drone, it comes with two things: the software on a DVD, and the InterLink Elite controller.
Because the InterLink Elite is the same one that RealFlight 7.5 uses, you can purchase the RealFlight 7.5 Upgrade, which is software-only, and use the InterLink Elite you already with RF7.5.
The end result is two completely separate software installations.
It is important to note that you cannot then sell the RealFlight Drone software by itself and continue using RF7.5 with your InterLink, because that controller is an indivisible part of the RF Drone package. You can sell the InterLink and the software together, or not at all.
RealFlight Expansion Packs and Mega Packs are content-only packages that install for RealFlight 7.5. They are not standalone pieces of software, and they are not compatible with RealFlight Drone.
You can use your Spektrum radio with RealFlight if you like by connecting it to the back of the InterLink Elite. There is plenty of information elsewhere on the forums about how to do that and what to watch for, so I won't go into it here.
Unfortunately the RealFlight is no longer offering the Upgrade 7.0 or 7.5 (software only) package, through their retail connections.
Its listed as Discontinued.
I would imagine Tech Support offered the RealFlight 7.5 Wireless SLT Transmitter Interface Edition, as the cheapest alternative, to ensure you got Legitimate software.
The SLT Wireless Interface works with SLT transmitters, or standard transmitters equipped with the Tactic Anylink (2) SLT Adapter.