A little contribution to the Transportation forum data base here!
I just purchased train tickets for our upcoming trip in June. I have found seat61.com to be a virtual encyclopedia of train travel in Europe, and it recommended loco2.com and trainline.eu as two sites for booking with Renfe/SNCF that do not carry booking fees. I want to report that loco2.com was easy peasy to use, and indeed had no booking fee, but trainline.eu (also easy to use) does ( a very small one for this trip). Otherwise the price was the same. SNCF releases seats 119 days before travel on this route.
I also purchased tickets for the EC train from Munich to Venice which we'll ride earlier in our travels, and used the bahn.de site, which was equally simple to use, no booking fee. Bahn.de releases seats about 6 months in advance of your travel date.
Hope someone else can use this info!