You can buy a reservation wherever you like: other websites, ticket office, etc. I try to avoid using Eurail/Interrail for reservations, since they have extra fees.

ÖBB normally charge only 3€ per person and train. DB charge 4€ per person and journey (which can include multiple trains). Interrail often want 8€ per person and train.

Trenitalia day trains can be booked via ÖBB using these steps (3€ or 10€).

For train with optional seat reservations like (ICE (except France), Railjet or EC or IC (DB, ÖBB, CD, MAV, ZSSK, SZ, ….) than you can book most of the time directly via the train operator or via Deutsche Bahn (4€/trip) or ÖBB (3€/train). 

For IC, Frecciarossa, -argento, -bianca and EC Italy-Switzerland, you can buy the reservation online via ÖBB using this steps. Please note that EC and RJ(X) Italy-Austria-Germany do not need a reservation but it’s highly recommended (it works like for german and austrian trains via seat only option) and for the italian part you pay a fee (about 10€, bookable via ÖBB using the same steps like for trains in italy via ÖBB or on board of the train for the same price). 

Thank you both for your thorough responses! Very much appreciated :) 

There are NO discount codes (I know this is a favorite US pastime), nor have there ever been. However-in some periods (mostly slow season) ALL passes are offered with 10 or sometimes even 15% discount-with more strict rules about no cancellation etc. If you only plan for long future, it may pay off to wait and check regularly.

However-about all newbees tend to make the very same mistakes and use the superfast trains for long distance over borders-the extra to pay can in total be quite high. You save more if you plan more carefully&wiser.

In the distant past some US resellers would offer a tiny discount, which they in fact gave you as a small cut off their own commission-but since most passes are now mobile app, this funny habit has about died out.