Good luck with it! Definitely contact the seller first and explain the situation, most sellers are more than happy to refund/exchange items and postage fees providing the buyer contacts them first and is civil in their dealings (Which I know you will be!)
A lot of sellers I know (just friends who do it as a side business or hobby sort of thing) get really annoyed when a dissatisfied buyer just jumps straight in and opens a dispute or gives negative feedback without first asking if the seller can do anything to set the wrongs right - they've not had a chance to fix it up or defend themselves and they feel it's unfair.

I really hope the seller is understanding and accommodating! It's very annoying not getting what you paid for, on the few odd occasions it's happened to me the sellers have been very understanding and prompt in dealing with my complaints. Fingers crossed for you that this seller is one of the good ones