My opinion is this;
The seller did you a favour by halving the initial postage, as this is what you should have paid. Whether it's expensive or not, I've never known a seller to go halves on the postage. Unless, it did only cost £15 or so and he got another £17.50 out of you
If the lights were not as described, you were right to ask for a refund and he did the right thing by giving you a refund even if it took 5 weeks.
The return postage is your cost unfortunately. If you buy something on ebay and raise a dispute with paypal, you can only claim the cost of the item and the original postage, not return postage.
To be honest, he has done you a massive favour by refunding you before you have returned the lights, not many people would do that....