If you were to get a bid with Company A for a new driveway, and you agree on $5,000 if they also include a mailbox repair. Contract signed. Company A subs the work to Company B, Company B subs the work to Company C, and Company C subs the work to Company D. You get your new driveway and repaired mailbox, but Company D charges you $100 for the mailbox repair. All they show you is their contract from Company C that says to charge you $100 for the mailbox repair. And since they aren't Company A (or even know anything about them), the contract you signed means nothing to them.
I find it laughable you would go any further than just dealing with Company A to be made whole. I can't even believe there is discussion about it.