notice to my client for not accepting the shipment

I want to send a legal notice to one of my client who had ordered some products from our company back in 2016 and never come back to collect the products, the products were ready and was lying in my warehouse wherein I paid huge warehousing cost, in 2019 March they suddenly came and started chasing for the refund of their amount, I told them that the products were ready and no refund was ever promised or planned the only solution I have is to send the product which was tailor-made on their special requirements, I shipped out the products via a sea container which is waiting for their clearance and pickup, which they are not receiving, I have been charged heavy damage cost and already paid for shipping, I want to write to them that if they don't accept it in the next 3 days the shipment will be taken back and destroyed and thereafter there will be no refund or compensasene given