If the OP and seller both made the transaction in good faith, and the seller accepted payment, then in my opinion the seller should honor the transaction.
I'm not sure what the law is here, but if the seller made an error, it's on them. When I have sold stuff (not as a retailer, but as a private party used sale), I consider the item the property of the buyer once I accept payment. I have sold things in the past where I paid shipping, and I have had the shipping cost more than I expected. I just paid the shipping cost and never even mentioned it to the buyer.
I expect nothing less of a business. If I had the same experience as the OP, I would allow the seller to cancel the transaction (since that seems to be what they want) and then I'd be done with the seller forever.
I understand the point you are making . It is quite unfortunate that the OP had this sad experience and the vendor should have put their house in order.
However, I don't think I agree with you that the order must be shipped to the detriment of the vendor if they discovered a shipping fee error.
You gave an instance of how you had to pony up the cost when the shipping fee was more than you expected . But there has to be a limit to the additional cost you can bear. What if the shipping fee turned out to be more than the cost of what you were shipping?. I bet you would have cancelled the deal or at least told the customer. That's an extreme example but you get the point.
Frankly, the margin of profit on these astro items are so small that asking a vendor to absorb the cost of a shipping fee error for an international shipping to that tune might mean losing on stocks they are even yet to get.
Most vendors in the U.S don't ship internationally and for those that do, it is few and far between. Sometimes, this kind of error occurs. I once purchased something and after waiting for 5 days for it to be shipped, the vendor asked to cancel as the the shipping company wouldn't play ball. I felt sad and frustrated but I wouldn't expect the vendor to ship at a huge loss.
I feel the OP's pain and it is such lousy way of doing business by the vendor and the OP was indeed harmed in more ways than one but two wrongs don't make a right.
The vendor must know that if it continues doing business this way, buyers will take their businesses elsewhere.