The seller is allowed to calculate the international shipping inside of ebay or outside of ebay directly at the post, whichever they so choose as long as the seller is able to weigh it and know the size of the package.
There is nothing that will ensure that a domestic and/or international seller will ship in the method that was in the terms of the auction and for which the buyer paid. I have had a few sellers charge priority prices with extra fees for insurance and they have shipped first class or parcel post without insurance and/or proper packaging. This does not happen frequently, but it does happen. These sellers get left less than 5 stars on their feedback.
I have also sold things internationally, as well as domestically, and underestimated the shipping costs. I happened to absorb the extra cost that was not stated in their auction terms without penalizing other buyers by inflating their fees.
The best thing to do in a listing is to offer multiple shipping options to the buyer to let them choose what they want to pay for the post. I also provide the USPS website link in my listings so they know that it is actual cost. I have no issues offering them multiple shipping options and re-invoicing them; however, it needs to be communicated to the buyer that first class/parcel post can take several days/weeks/months longer than priority and does not always offer the same automatic benefits as priority. It is a cost/ship time trade-off.