Before you go down the road of opening a claim.
Firstly what's their feedback score, if it very high give them the option of rectifying the situation at their cost before the threat of leaving negative feedback and opening a claim. I had a few situations of incorrect items delivered and at all times I got a positive result as I gave them option of correcting the issue without me leaving negative feedback, postage was always paid for by the seller. I might have been lucky but genuine sellers will try avoid negative feedback at all costs.