I'll go ahead and shed a little light on my experience of doing what you're trying to do.
-I ordered the STI (2012) wing off of Subaru's website for about 350$ Shipped (shipped to the dealer)
-They had it painted for 250$-300$ if I recall correctly and were going to install it for about the same price, so all in all a little under a grand for the entire thing. Seems like a lot of money just for a spoiler huh?
Well come install day I dropped my car off and get a phone call a few hours later telling me that there is a problem. The WRX trunk doesn't have the same supports as the STI trunk. With that said Subaru said they would install it if I really wanted them to, but there was a high chance that eventually or overtime the weight of the spoiler would bend my trunk. When ordering the wing they weren't aware that the supports were different and assumed the whole install would work fine. Luckily they game me my money back due to them not having all of the proper knowledge about this process.
People on here might tell you you'll be fine and just put it on and of course you can if you wish to, but I just bought a 26k car and I'm sure as hell not taking a chance of messing it up.
A new STI trunk was like 800$ or so, and that's without paint and install. That 2k price the dealer gave you is probably for the trunk,wing, paint, and install.