Standard International Shipping is a generic way of saying that the seller will ship to your country and they will decide how to ship when the order is made.
Today I shipped seven packages using a similar term (Economy International Shipping) all of which were also sold as Free Shipping. Six of those were shipped as LetterPost without tracking and one as Tracked Packet USA for $12.46.
None of the buyers were charged shipping. That had been included in the asking price.
Sellers cannot raise the cost of shipping after purchase.
Sellers must ship to the address the buyer used when they bought.
The whole thought process of your seller is puzzling.
Were you given a price for shipping? (This would include Free Shipping.)
If not, the seller does have the right to charge you whatever price is reasonable.
If they did, you accepted that price (including Free Shipping) as part of the transaction and the seller cannot raise the price.
- Neither can the buyer reduce the price or suggest a cheaper method of shipping, by the way.
But rather than argue, I would agree to the cancellation and leave appropriate feedback.
The most effective feedback is calm and factual. "Wanted me to change my shipping address and raised advertised cost of shipping. " perhaps.