Not a problem with the seller

You say you are using the app

The app is widely and long known to show 'free' postage when it is not free postage at all

What case do you want to open? there are no grounds for one

It is not the seller's fault the app wrongly shows 'free' postage

The seller is not obligated to accept a cancellation request

It would be the same if you had an item listed for sale, a buyer who was using the app saw 'free' postage on that listing when it is not 

Feel free to buy from whatever site you wish

eBay Help is at the top of very page