According to a screenshot from a buyer in Quebec an offer includes the following message:
"Il se peut que d'autres acheteurs aient recu cette offre. Le premier qui l'accepte remporte l objet"
Other buyers may have received this offer. The first to accept it wins the item
No note that shipping is extra. There is a disclaimer to not contact the seller to negotiate offsite. Unable to see if there any message under the accept/reject screen in the screenshot.
If this is all a buyer sees, this could be what is causing any confusion/cancellations? And the same confusion for any International buyers ( Where a translation is involved).
-Lotz