Book with Train tickets in Europe - Search & Book on Rail Europe.

There’s no fee. Choose “add rail pass” (desktop version only).

Are you referring to a Eurail pass? I'm travelling with the Interrail pass from Brussels to London. I need a seat reservation for that journey. I am doing the booking on the app on my phone. 

I am referring to an Interrail pass because your profile says: Switzerland. 

You may do your seat reservation from Bruxelles to London on the page mentioned before without a fee. You’ll get a ticket and you may print it. 

If you’d like to pay an additional fee you may use what ever you did use before (but you didn’t even tell where are you trying to book). But I thought you did ask how to avoid the fee?

The price for the reservation itself is 30 € 2nd class and 38 € 1st class. When booking with interrail itself there’s an additional booking fee of 2 € per person.

You can always get those reservations at a SBB counter without fees. However it'll cost 30 CHF so you lose a bit with the exchange rate. At least that's what I paid recently. :)