DB struggles to sell standalone mandatory reservations. Until this year it was simply impossible and now it seem to be possible but as you said they sell them separately...

Better to book through ÖBB: 3€ per train. There's no seat map though. You have 3 min after payment to cancel the order if the seats don't please you.

I mean it doesn't matter whether you book Frankfurt - Amsterdam or Bern - Frankfurt - Amsterdam. In which way are you screwed?

My prefered itinerary is Basel - Cologne (buffer) - Amsterdam.

Did you proceed to the payment page?

There seems to be an error showing the double price on the first page when a train with temporary mandatory reservation is involved, but when you proceed to the payment page, the price is correct. 4,90 EUR in 2nd class and 5,90 EUR in 1st class.

Yes that seems to work!

DB normally charge mandatory reservations separately, so I'm not sure if the €4.90/5.90 price is a bug or a feature...

The € 11,80 seems to be a bug. This Tuesday I finally managed to book a connection with changeover in Frankfurt, and at first it seemed as if I had to pay € 11,80, but then I checked out and had to pay only € 5,90.

Probably unrelated: when I tried to book a seat a week in advance, the train Amsterdam-Frankfurt was fully booked. One day before the trip there was suddenly quite a lot of place. And then the train seemed to be much less crowded than normal…

It’s still slightly annoying because I can’t be sure that there will be place one day in advance.