Hello, yes this is pretty common when listing on .ca and bidders thrying to bid on .com
I usually tell the potential buyer/bidder to log out of his/her account and then close the browser they are using. Open new browser then log back into their account and try again.
From past experience this works for most buyers in th USA. It appears there is yet another bug in this antiquated software platfrom that e-bay offers its buyers/sellers when it comes to software identifying potential buyer/bidder location(zip code) since this only happens with listings that offer calculated shipping.
The first few times buyers messaged me with this issue I actually contatcted ebay reps only to be told that there was no issues whatsoever with our listing in questions...lol.
