I think the best way to do it is tell them an amount for you to post it back.
Don't bring your pending order even into the equation.
For example if you want $200 to post it back tell them that is how much it will cost and get the money upfront.
Work out the amount of discount you want, cost up the postage add them together and tell them that's what you want.
Work on the way they work, no money, no goods.
How are they going to come and collect it if you don't post it?
Make it hard enough for them and they will write it off, they can only spend so much time on it before it becomes too expensive to chase.
Their best bet is for you to do the running around for them.
What's the worst that can happen, they wont sell to you anymore?