Can I send a legal notice for refund dispute over courses?

I purchased four courses from a website offering a 90% refund on completion of the course within 90 days. One of the courses’ timing conflicted with another course, so I requested them to change my batch. However, they didn’t inform me that the 90% refund offer would not apply to batch changes. I missed reading this in the email, and now after completing the course within 90 days, I am told I am not eligible for a refund due to the batch change. Additionally, there was a complimentary course with the purchase, but now they say the complimentary access has expired, and this was not mentioned before purchasing the course. I want to send a legal notice. Can I do that? What should I do next, as they have denied my request via email?