empowerment

So, I hate to sound like a complainer, but two situations really igrinded my gears tonight... First, two scammers come through my one cashiers line with clearly fake coupons, being on the lanes myself, I told them I can authorize the limit of 4 coupons for a transaction for each of them,but...

soo.. im a new cashier and im a little confused as to what I should do when an item I ring up's price is questioned by a guest... for example something rings up as 15 and the guest says "I thought that was 10" something like that... do I change it and put the reason as guest price challenge? it...