I used (as in provided a service) 3d hubs for a year or so… maybe 2 years ago? The biggest problem were people giving me data that was in no way 3d printable. I just did it for fun, but based on my time involved,… it just wasn’t worth the money. I’d have hours of back and forth with people trying to explain why they had bad data.

So, to answer your question, there almost needs to be an intermediate step where somehow… the client is made aware of the 3d printability of their data. Maybe that alone is worth an hourly rate for “data inspection, and cleanup” ? Having that as a listed step that costs cash money preps the client for that expectation . Otherwise, most people figured they could just throw any 3d data over the fence and the magical 3d printer would do the rest.
Also, their model for figuring out how much a print would cost was wayyy to generalized. Individual hubs need much finer custom control over those knobs.

I’m happy to print your stuff, but I have little interest in redesigning it to be printable, unless they want to pay a premium for that service.