Cost of seat reservations on Bernina express

I live in the UK and am visiting Switzerland October 27th - November 1st. I have bought a 1st class Swiss Rail Pass. I plan on taking the Bernina express trip - from Chur to Tirano and back to Chur.I am looking to make a seat reservation. I have looked on swisspasses.com and their price is £20.50 per person (with 1st class swiss pass). The cost on the site www.rhb.ch is 18.00CHF each (1st class) Do you know why is there would be such a difference in price? I am unsure of which site to purchase the reservation from - the obvious choice seems to be the cheaper one - but am I actually comparing like for like?

Just to clarify my question. Because the prices seemed so different, I was wondering if the seat reservation sold by swisspasses.com was different from the seat reservation sold by the site www.rhb.ch? I am so confused by the different web sites. Am I correct in thinking that what I need to travel on the Bernina express is my Swiss Rail Pass and a seat reservation. (and it doesn't matter where I get that seat reservation from?)

Hello Pinky! Prices can always differ per reseller, depending on (for example) exchange rates. You can just go for the cheapest option. We don't work with swisspasses.com, so I can't provide information on their reservations.

Anyway, you need your Swiss Pass and your additional seat reservation for the Bernina Express trip indeed, and it doesn't matter where you got your reservations from - as long as they're for the exact train and couch number you're entering.

Thanks for the clarification!