Second Hand goods when I paid for new items

I bought an 8 seater outdoor dining table and chairs with cushions. The table, 8 chairs and 4 cushions were delivered. I paid for 8 cushions. The response I get from the store is that it is not their problem and they cannot help if things go missing. I have requested that they send me new cushions that they owe me, but they insist that they cannot do that. How is the fact that they cannot deliver my items my problem. I paid for the item and delivery and I should be entitled to it.