fwiw, slip.so appears to be pivoting away from education marketplace. I received an email for a course I pre-purchased recently:

"We're pivoting our company away from the course marketplace idea, so we're reaching out to inform you about your purchase and what to expect next.

We'll be helping [redacted] migrate his/her content to a new platform, and make sure that if you've purchased his/her course on Slip, you will get access to it on the new platform, with no additional charges.

You'll receive instructions via email on how to access the course on the new platform."