My husband and I have had many problems with this,
The gas station right after Capo, next to the McDonalds has given us issues. They ALWAYS try to buy the coupons from us, and then they always try to put in the wrong amount as well. I mentioned something to the people at the NEX when purchasing more coupons, they just said sorry, just keep an eye on it and that was about as much help as they were... Its super irritating to go to a gas station and get ripped off or harassed each time. We seriously spent about 20 mins trying to get this guy to leave us alone, put the correct amount in and stop asking to purchase the coupons. See the thing is ... they speak English well enough to have conversation, until you have a problem or spot them doing something, then suddenly you get sorry English bad. .... I mean I guess there is no way to control it but just be aware as everyone has said.