Seat reservation for Interregio (Zurich Airport - Basel SBB)

Hi there, I bought train tickets between Zurich Airport and Basel SBB at a travel agent. This tickets do not include seat reservation (means I can take any trains while the tickets are valid).

I knew that ticket between Zurich HB and Basel SBB enabled me to reserve a seat, but I prefered the direct train from/to the Airport and this route did not allow me to reserve a seat.

I am worried if I should pay more on the train.

I believed upon purchase that this route was free seat only, but it looks possible to reserve a seat when I check SBB website (I did not reach to the seat select page tho)

Any tips are appreciated.