Is it for the IC between Amsterdam and Brussels ? Seat reservation is indeed not necessary or even possible.

There's no supplement, whatever the app says

This is between Innsbruck and Verona OBB train ECB 83, tomorrow at 11.24am.

Then there is a 10€ surcharge for the direct ECs through Brennerpass. You can make it on tickets.oebb.at : add Interrail/Eurail as a discount and select one-way tickets.

A seat reservation costs 3€ extra. However it is impossible to make one for tomorrow's train at 11:24 so I expect it to be pretty full. You can always board but you might have to swap seats along the way or stand for part of the journey.

Alternatives :

- earlier or later EC

- regional train to Brennero + RE train to Verona : seat reservation not possible and no surcharge

This advice is appreciated but very difficult now as it is late !!

I'd say try your luck on the 11:24 train, 1st class carriages shouldn't be full. However please note that the surcharge costs 15€ for 1st class passholders.

For all Italian trains also use tickets.oebb.at (no booking fee) : 13€ Frecciarossa, 3€ IC trains.