Well, unless you want to air out all the dirty laundry (all your emails, and all theirs) and stand aside to be judged by the whole internet, you're going to have to make up your own mind about just how mean you are feeling. Do you really have to pay someone to pack the bike up? How unpacked is it? well, whatever. It has to go back.

How about this: tell them you needed a bike, and you went out and got one, and though it's not what you wanted it will do. So you have already spent the money they are about to refund. If they still want your good graces, they are free to send you another bike. After they refund all your money.