Jerry is the typical moron who spends more money than he has and ends up in deep trouble. It's not explained in the movie, but typically money is spent on drugs or gambling, or in purchasing expensive gifts for a lover. To cover these expenses, he steals from GMAC, taking loans against non existing cars, which is why he sends illegible VIN numbers to the lender.
The Tru Coat is just the usual add-on, high-profit item sold in car dealerships. Also called "undercoating" it's something where the salesperson earns a commission of 50% or perhaps a lot more because it's something essentially worthless.
When I was young they tried to sell me something similar. Once it was an "auto theft prevention registry." On another occasion it was an extended warranty for the electronic components. In every instance I said "no" very curtly. If they insisted, I'd get up and say that it's getting late and that we can continue the next day. That was enough to make those charges magically disappear!

In the off chance you are being serious:
The Tru Coat thing was just a standard car salesman scam. You talk about how the it can save thousands of dollars (which it can) and that the buyer can get it for $500. Of course, you don't tell him that all it really is is just a quick spray with a $10 can of undercoating.
It's a pretty well-known scam that only rubes fall for but it's something that someone like Jerry who was desperate to get money would be prone to trying to pull.

Check out the meaning of the term
'authorial intent'
.
Your "theory" has no basis of fact within the film.

Intersting . But I'm pretty sure he was pilfering money through the business and they were closing in on him .

Interesting theory!!
.... TxMike ....