Shipping iMac for trade-in

I bought a new Mac Mini combined with a trade-in of my 27" iMac. Apple has now sent me 4 shipping boxes for a 20" iMac. Apple has canceled my trade-in because they haven’t received my iMac, which I can’t send in a too-small shipping crate.

I need to know who to talk to at Apple so I can get the right shipping crate and have my trade-in honored. Calling Apple clearly hasn't worked.