I think everyone forgot to answer the question: a Cert from a Certificate Authority (CA) proves you are who you say you are by way of a trusted authority (like Digicert, not Comodo) checking you credentials. A self signed Cert does not have this process, everyone is taking your word for it, and when making a credit card purchase this can be a deal breaker. That said, both certs are secure if they are generated using the latest accepted encryption standards and correctly implemented. This can provide an encrypted communication nobody else can see. No difference between them except credibility and accountability. "Let's Encrypt" certs are fine if you trust the person communicating with you. With a CA issued cert, they would probably get caught and distributing malware or skimming credit cards and threat actors would need to rely on infecting a legit website instead of providing this service for you directly. Without a CA, malware could be distributed directly but over a secure connection. Just because it's encrypted with a Self Signed Cert doesn't make it safe to download!
