Before you jump to conclude that a company is trying to intentionally rip you off, be sure to keep the quote above in mind. It's a good way to treat people in general, as you want to be very careful before accusing anyone of intentionally trying to cause you harm. When you get jerked around it can sometimes seem that it is occurring on purpose even when it is not. I speak from personal experience on this matter.
Large companies are often bureaucratic, inefficient and poor at communicating.
Problems with them are common, but bad intent is not. In most cases when there is a difficulty with a vendor it is not because the vendor wants to screw you around but rather because they have poorly trained employees, they are short on staff, their computer system is ineffective or there are other issues. These aren't valid excuses for improper behavior, but they are also not a reason to assume that someone is out to get you. Be sure to give the company all reasonable benefit of the doubt before you give up on them and assume that they are acting poorly on purpose. Recognize any efforts they make at reconciliation and try to use those as a lever for further negotiation.