Slightly off topic but the experience I've had with the last two attempts to recycle phones with companies have put me off completely.
I've had similar experiences to the OP where the offer by e-mail was about half that of the original quote with various "reasons" given for the discount.
The best being that they found "significant" scratches to the phone case which when photographic evidence was provided turned out to be the slip-on case I sent back with the phone to protect it. Rather than raising the money back to what I was originally quoted, their "senior technician" re-evaluated the phone...and "found" some more scratches (invisible on subsequent photos) to justify the reduction in cost.
Unlike the OP, I just gave up and took the money rather than go through the hassle of trying to get my phone back. It's put me off the recycling route for future phones; they're hardly worth anything now so I'd better off donating to charity and letting them deal with the hassle or going back to having a drawer full of old phones in case my current one breaks...