Not a problem! Simply refund the customer's secondary transaction. See our article on how to refund transactions here.
Overcharging or Undercharging:
You'll need to refund the transaction in full and reprocess it again for the correct amount - we don't currently support partial refunds. See our article on how to refund transactions here.
If a customer accidentally tips, refund the transaction and reprocess it after the customer has adjusted their tip setting on their app. You can also have the customer contact firstname.lastname@example.org for assistance.
Have customers email email@example.com if they think something looks off in their transaction.