You should have asked for a postage quote if the listing did not have an actual amount.

It is hardly ebay's fault if you committed to buy before asking the seller for the cost.  And yes, you have committed to buy even if the postage is outrageous.

I cannot comment on the other incident as I don't use the mobile app.  but I would suggest that if you have misused the "Request Total" function then you have also misused the other function too.