If the merchant for some reason does not charge you enough you can pay them the difference in cash or card.  

If the merchant entered the wrong amount to be paid via the app and you pay via the app you may contact us with your bill from the app and we can have this rectified. Please email: [email protected] 

If you have been overcharged and payment was made directly to the merchant and not via the app then the merchant would then need to arrange the refund with you in cash or back to your card.