Providing HTTPS certificate for your customer domains

Some product provide user custom domain feature.

Someone making these product said that serving HTTPS certificates for their customer domain is annoying.

They said it is not hard to make custom domain feature. But providing HTTPS certificates is not easy. I looked up many Let's Encrypt clients for a few days, and I found all of them cannot provide certificates dynamically. I know they can make it with more time.

I also found some products that already implement this idea. But I can't understand why their prices are that much expensive.

How do you think?