Zero confirmation payment is impossible to implement. Because when you use blockchain you can just use double spend.
So just because people have the ability to perform a double spend, it's impossible to accept zero confirmation transactions? What a logic....
So the confirmation is unlikely to come then the amount of bitcoin back to the sender wallet. Using zero payment system is at risk to apply
First of all, most merchants make use of a third party payment processor. In some cases merchants can choose to allow transactions to be accepted without confirmations, but that's only the case when the transaction in question has a proper fee included. If you think you can troll their system by including a tiny fee so you could later perform a double spend, then you'll just need to wait for your transaction to confirm, otherwise it won't get accepted.