Most likely you're not going to have to go to Retiro -- you will have to go to Ezeiza. Unfortunately with Fedex any packages (especially artwork) that have to go through the aduana -- and this certainly will -- you now have to go out to Ezeiza. Fedex instead sends you a ltetter that I think they offer brokerage services, perhaps not, I can't remember. Basically when my mum sent a package that required me to go to Ezeiza, pay brokerage services, pay a bribe to get package released etc I decided it was worth neither the time, the hassle, nor the money.
I would not recommend sending packages like this again -- you could potentially end up paying thousands of pesos in transport and brokerage etc.
Hopefully someone who has actually been bothered to go through the process will have some good tips for you. I am sorry in advance for your pain! Suerte