You mean passholder reservations like on TGV/ICEs to France?

Yes those can be bought at DB ticket counters.

I guess that the question is about the current sale on Interrail and Eurail passes that ends on the 18/7.

The sale is over now, but I have seen that 20% discount promotion also being made by many national train companies. 

As an exception, I saw that for Sweden their national railway SJ is not selling Interrail themselves, but refer to the Interrail-website. 

The price is the same and I believe such things only matter in case of complaints or claims, as you would need to address the firm that sold the ticket to you. Having said that, my impression is that the Interrail-company is busy, but willing to help. My impression after multiple claims to DB is that they are burocratic and old-fashioned when it comes to contacting them: by phone only.