I keep getting this same message when I search for a simple return yet it works when I do them as 2 x one ways (tried multiple times on multiple computers, cleared cookies etc).

I wanted to book it as a return to avoid paying 2 x booking fees but I also want to avoid calling the call centre to book if necessary - will they charge me more to book a return in booking fees if I call through the call centre, or should I just pay the extra $50 and book it as 2 x one ways to save time?