The figurines you have purchased should be able to be combined.

 The seller states that they use padded bags for postage, so that is a cost on top of the post stamp charge. If the weight of the extra figure pushes the package into a higher weight range that could increase the cost. Ebay charge 10% commission on postage costs, thats an extra cost. The seller has to actually take a few minutes to find your figure, time is money.

Having said all of this the seller should respond to your messages and if advertising combined post, should do so.

The sellers feedback is excellent with perfect stars for communication and only a slight ding for post charges. It may be that they have a family emergency or work commitments and have not been able to get to the computer. It may pay to wait before getting too annoyed in case there is a reasonable explanation, given the sellers very good track record.