I used justly to book a cheap "business class" ticket on Norwegian. My frustration was I could not choose my seat anyway, anyhow! I contacted Norwegian and they told me since I booked my ticket on another website I couldn't do it. I went around and around. Eventually I was able to select my seat when I was at the airport 24 hours ahead of time.
I found Justfly to be very responsive to my emails. I think the problem of choosing a seat is typical to ANY website that isn't the airlines. My fare was good, it turned out it was not a "business" class, but more premium economy as that is all Norwegian offers. At any rate, the flight was good. I would use justly again.