It would probably be a crime if you manipulated their computers to accept the $10 price. But if you submit you want a plan for $10 and their servers just accept it then what crime could I have committed?
You've still committed a crime even if the other party doesn't actively prevent you from doing so. You can walk into a store, and walk out the front door with a television and probably nobody will stop you. Additionally, you can walk into a bank and demand that they give you money, and they will give it to you.
In none of these cases are the companies condoning or supporting your actions. You can try the argument that the bank teller gave you the money and that it's bank policy to do so. But it probably won't work.
It's strange to see people bending over backwards to deny that this is a crime. Surely if someone manipulated your bank's computers to wire transfer your entire balance to another account, you wouldn't be asking, "what crime could [they] have committed?" You'd feel like somebody stole from you.
I think the reason people are denying this is a crime is because it's happening on their machine, not a remote server. And the remote server is presumable accepting the request.
Using your store analogy it would be like bringing a television to the cashier and telling them you will pay $10 and they accept.
