Looked into this a bit more for you Kup and Ebay's stance is once a bid has been placed, they may not get involved in postal issues as you appear to have accepted the terms of the sale.

Having said that, you could try going down the path of making a complaint to the TRUST AND SAFETY section of Ebay as this department deals with bad items or bad sellers and they will investigate the overcharging of postage and the profiteering issue relating to the seller.

I would not be leaving negative feedback at this point as its still early, but that's completely up to you. If there is a chance you can get reimbursed, I would keep things civil for now.