I think that when you right click a voucher, you should get a message if you really want to use it and you can right click again to use it. Kinda like [Slot x] shop signs. This morning I was buying a voucher from somebody and they accidentally misclicked my shop sign and ended up using it so they lost it and didn't want to use it.
We got something like this yesterday, but it isn't possible to have a message pop up when you're about to do something. IcecreamCow and Aikar said it. You'd need a mod and requiring everyone om ENC to get one mod so they can play is a little pointless.
Then how do you explain [Slot] signs? They have a chat message sent to you as confirmation. It should work just like that. It's not a pop up, just a chat message like slot signs.