How To Reply To This Legal Issue/notice To The Customer.

Hi, I am a founder of an internet e-commerce marketplace that connects Indian customers to international makers of unique consumers electronics gadgets and other unique products. We had a customer that ordered 6 items worth Rs 69000 last year. Out of which 4 items worth Rs 24000 were delivered to the customer in the last year itself but the other 2 remaining items faced some problem during importing and shipping from the brand. After a delay of few months, the customer asked to change the order and replace the 2 remaining items with 2 other items from our catalog. Out of which one item has been delivered to him. After receiving the item, the customer complained that he wanted the item that was priced at Rs 9999 and not the one priced at Rs 19999. While we clearly mentioned to him multiple times that the item Rs 9999 was not even available on the website and the item he mentioned in the written email to us is worth Rs 19999. At the same time, the other remaining product that the customer had ordered as a replacement is also underway. Now the customer has sent us a legal notice claiming of defrauding and asking for Rs 38000+ Interest. We have the copy of all the conversation between our team and the customer as well as confirmation of goods received. The last delivered item and the remaining items together makes up to Rs 62000 in value. We even informed the customer that we will write a cheque back of Rs 10,000 for the inconvenience caused to them but instead we have received the legal notice in return. Please let me know how we should proceed with this.