Amazon wont refund because they never collected as arranged

I requested a return on a backpack. I printed return slip and packaged. They never collected. Now they refuse to return it. No where on the return on website is there a place to follow up on pick up. I contacted customer care for another item query and asked about this return. This is unacceptable. They slipped up and it is costing me thousands of rands for an item I can't use.