I'm not saying Kanter added additional cost on top of what was originally quoted.
I read it as Kanter quoted the job at $225 and during the course of repair they found additional damage that added $100. So now we are at $325. Figure shipping and you get the $340.75 OP was charged.
Everyone was happy until OP found out what Kanter actually paid someone else to perform the work and realized they had only been billed $146.44.
There’s no getting around that Kanter is only billing what he agreed to pay.
But, as long as people are willing to pay then more power to Kanter. They apparently offer a service that's worth that kind of markup to enough customers to stay in business. This is a prime example of knowledge is power. If the OP knew who to send it to he could have eliminated the middle man and saved a decent chunk of change.