Sounds like the seller did his best to estimate the shipping based on a previous sell. Remember the seller is probably not a shipping expert. The plane could have come up as oversize. He wouldn't have known the cost until he actually shipped.

You'll have to decide if the total deal, including the extra charge, is a good deal on the model. You certainly have the advantage in this deal. If the roles were reversed, how would you react. If getting the model $7.65 cheaper gives you a warm fuzzy, go for it. I suspect that if the seller had initially quoted $17.65 for shipping, you'd have made the deal anyway.