I wouldn't move on. I would fight the friggin' thing until I was blue in the face. You may be happy to rollover and play dead in this situation, but I think the OP has a very good case if the particulars of the account have not been spelled out to him by the sales person. Whenever I have bought bandwidth, the details have been made very clear to me, whether I have communicated by email, or filled in online application form.
The act of selling a very expensive uncapped account 3 days prior to the end of the month when they could just as easily have sold
a cheaper 15 day version of the product is indication enough that their intention was to rip the OP off. Who treats new customers like that??? It disgusts me that people on this forum think this kind of thing is acceptable. It is not. It is a disgrace.