JS on Smart Investor (video interview), page-249

What is your source for the claim "the remaining $8k they can do what they want with"?

In their detailed description of the financing arrangement, there was no mention of the customer being able to use the line of credit for other things.

We all know there is no such thing as "interest free". In the case of Big's subscriptions, they need to mark up the cost to the customer by 31% in order to cover the cost of the interest.
( if big wants to receive net $1 for their services, they must charge the customer $1.31 then pay interest @ 24% , $1.31 x 24% = $0.31 )