Has anyone had any experience with them moving their personal stuff?

Heres my situation and I would like the opinion of someone who understands the rules of the industry.

I had to move about eight boxes, and a fairly heavy wood bedframe <no mattress> from CO to NC. Most estimates came out around 1.3k. Wich was not worth the value of what I was moving so I decided to opt out and just leave the property.
This company called me, I explained to them the previous estimates and that I was not going to move for that much because of the value, blahblah.

So the agent says he can do the move for alittle under 800, I say this is fabulous and I sign a contract. Actualy two because I have two moves, but thats besides the point right now.

Now when the movers show up, not only do they make inclinations that I have to pay extra for the offload, but they contact someone on the phone while at my house, who claims to be dispatch and asks me to give them 200 in cash. I refuse, because A: not a chance, and B: Who just happens to have 200 in cash on them?

Now Im getting a call saying that my weight is actually 1k over what was estimated and I owe them an additional 500 at delivery, which brings the total up to what I said I didnt want to pay. In the contract it does state that overages will need to be paid and that the initial weight estimate is just that.

So my question is, and this is to those who actually have some experience in either the moving field or law, how much of a chance do I have in fighting this? And how can I get proof as to how much my property actualy weighed? Because I have my doubts it added up to 2.5k lbs