How do these services work? Do they need access to your credit card or bank account? Do you EFT money to it?
Install app on your phone.
Link your credit card to it. I don't know if you can link any other type of cards.
Then take a pic of the image that is linked to the store or restaurant.
Enter the amount.
Hit Pay.
Then the merchant receives a message of the amount.
It does seem that Zappers image on the till slip is linked directly to the actual transaction. I could be mistaken. But I think the amount was there already. I just set the tip percentage and hit pay.
You can also set a password on all of these services. Or - my favorite - with snapscan and an iPhone - you can link it to the finger print. so one less password to remember!