You do not need a reservation for this route. Seat reservations in Switzerland are unecessary and a waste of money.
Hop on and sit wherever you want. That train will be empty at 7am on a weekday.
In general don't book through Eurail (extra fees being the main reason). Have a look at https://www.seat61.com/interrail-and-eurail-reservations.htm
Btw this train is not operated by Rhätische Bahn but by Zentralbahn (not that it matters).
I have similar question, I am going to purchase ticket from Lucerne to Interlaken, the seat reservations says Deliver by post. I am located in Muscat, Oman and will be travelling in another 2 weeks. Is this email option possible for me as well ?
Train: Rhatische Bahn | PE 2914 | 2914
Date: Tue, May 14 7:06 AM – 8:55 AM.
Another question, incase if sent my post how much time it will take ?
No that doesn't work that way. But you can call SNCF (Press #85 for English) and receive it via e-mail. It'll also save the booking and postage fees.